REVIEW OF TEST MATRICES AND CONDITIONS
Roger Bligh
Texas Transportation Institute
Texas A&M University System

NCHRP Report 350 contains recommended test matrices and impact conditions for evaluating the impact performance of roadside safety features. This paper focuses on related issues that may warrant consideration in future updates of these guidelines.

IMPACT CONDITIONS

In NCHRP Report 350, impact conditions are defined in the terms of vehicle type, impact speed, impact angle, and location of impact. For a given safety feature, there are multiple test levels, each of which has its own set of impact conditions. For example, there are six test levels defined for longitudinal barriers. The impact conditions for these different test levels vary in impact speed from 50 km/h to 100 km/h and in vehicle type from an 820-kg (1800-lb.) passenger car to a 36,000-kg (80,000 lb.) tractor trailer unit.

For test level 3 (TL-3), which is considered to be the basic test level, there are two required tests for longitudinal barriers (i.e., guardrails, bridges rails, median barriers, and transitions). The impact conditions include a 2000-kg pickup truck impacting at a speed of 100 km/h and at angle of 25 degrees, and an 820-kg passenger car impacting at the same speed and an angle of 20 degrees. The pickup truck test is intended to evaluate the strength and containment capacity of a barrier, whereas the small car test is a severity test intended to evaluate occupant risk.

For terminals and crash cushions, there are three test levels, each of which includes up to eight required tests depending on the nature of the device. For TL-3, there are up to five tests with the 2000-kg pickup. The impact speed for these tests is 100 km/h and the impact angle is 0, 15, or 20 degrees depending on the nature of the test. For example, tests on the nose of the system are conducted at 0 and 15 degrees, while tests into the side of the terminal or cushion are at 20 degrees. The matrix also includes up to 3 tests with an 820- kg passenger car with an impact speed of 100 km/h and an angle of either 0 or 15 degrees.

Impact Speed

Although Report 350 incorporates a multiple test level concept, the maximum impact speed contained in the test matrices is 100 km/h (62.2 mph). Since the publication of report 350, the national speed limit of 89 km/h (55 mph) has been revoked and many states have adopted speed limits of 113 km/h (70 mph) or greater. This change has raised questions regarding the appropriateness of the current test speed and whether a higher impact speed is needed for TL-3 and above.

However, when contemplating such a change, the consequences should be carefully examined. For redirectional devices such as longitudinal barriers, an increased impact speed might be accompanied by a decrease in impact angle such that the overall impact severity remains unchanged. However, the energy that would have to be managed by terminals and crash cushions in end-on, 0-degree impacts could increase significantly, resulting in longer, more expensive devices.

Impact Angle

Another issue is the appropriateness of the 25 degree impact angle which is currently specified in NCHRP Report 350 for length-of-need (LON) and transition tests of longitudinal barriers. The relatively severe angle is intended to evaluate the strength and containment capabilities of a barrier. However, in many tests with the 2000P test vehicle, problems have been identified not with the strength of the test article, but rather with stability and severity criteria. Tests intended to evaluate severity, such as those conducted with the 820C, are currently conducted at what is generally thought to be a more representative angle of 20 degrees. Since the purpose of a structural adequacy test is to evaluate barrier strength and containment capacity, should the 25 degree angle be maintained but perhaps a more stable vehicle selected?

Further complicating the issue is the fact that the impact angle for the strength test differs for longitudinal barriers (e.g., Test 3-11) and terminals and crash cushions (e.g., Test 3-35). Whereas an angle of 25 degrees is currently used for barriers, terminal tests at the beginning of LON use an impact angle of 20 degrees. Should the impact angle be reduced to 20 degrees to correspond with the small car severity tests and terminal tests with the pickup at the beginning of LON?

Additional precedence for the use of a 20 degree impact angle is contained in the 1989 AASHTO Guide Specification for Bridge Railings. This specification uses the same impact angle for both small car and pickup truck tests. In order to help compensate for the related decrease in impact severity associated with the use of the 20 degree angle, the weight of the pickup was increased by 400 kg.

Energy Considerations

When quantifying the severity of an impact, the combined affects of both speed and angle should be considered. For barrier impacts, the impact severity (I.S.) is commonly defined as the lateral component of energy of the impacting vehicle:

I.S. =(approx.) ½ (M) (V sinØ)2

where: M = mass of vehicle

V = impact speed

Ø = impact angle

If we use the existing impact condition of a 2000-kg vehicle impacting at 100 km/h and 25 degrees as a baseline value, we can examine the relative change in I.S. represented by other impact conditions. For example, as shown in Table 1, the I.S. for a vehicle of the same mass impacting at 110 km/h and 20 degrees is only 79% of the baseline condition. This raises a question regarding the potential affect on the structural adequacy of barrier systems if they are tested to a lower I.S. than is currently recommended. If a lower impact angle is desired, an impact severity of similar magnitude to that of the existing strength impact condition can be obtained by either increasing the impact speed to 120 km/h (74.6 mph) or increasing the mass of the test vehicle to 2450 kg (5400 lb.) and the impact speed to 110 km/h.

TABLE 1. RELATIVE IMPACT SEVERITY FOR SELECTED IMPACT CONDITIONS

Vehicle Mass (kg)

Speed (km/h)

Angle (deg)

Percent (%)

2000

100

25

100

2000

110

20

79

2000

120

20

94

2450

110

20

97

While an increase in impact speed for barrier impacts may seem appropriate considering the recent increase in speed limits, the affect of such a change on terminal and crash cushion design should be carefully considered. For end-on impacts with terminals and crash cushions, the impact severity is simply defined as the kinetic energy of the impacting vehicle. Thus, the energy that must be managed by energy absorbing devices increases with the square of the impact speed.

As shown in Table 2, if a 2000-kg vehicle impacting head-on at 100 km/h is taken as the baseline impact condition, an increase in impact speed to 110 km/h represents a 21% increase in the energy that must be managed. The two conditions shown in Table 2 that represent an equivalent I.S. for an impact angle of 20 degrees represent about a 50% increase in energy in head-on impacts. This can have a significant affect on both the length and cost of energy-absorbing devices.

TABLE 2. RELATIVE KINETIC ENERGY FOR SELECTED IMPACT CONDITIONS

Vehicle Mass (kg)

Speed (km/h)

Percent (%)

2000

100

100

2000

110

121

2000

120

144

2450

110

148

Accident Data

The existing TL-3 impact condition of 100 km/h and 25 degrees was selected to represent the worst practical condition that would be expected in the real world, i.e., it represents the upper end of accident speed and angle distributions. There has been no significant change to these conditions since their original selection and adoption the 1970s, and their relevance to current accident distributions is unknown.

Table 3 shows some results from the impact speed and angle distributions that were obtained from the Pole and Narrow Bridge studies conducted in the late 70's and early 80's. The distributions were based on reconstructed accidents. As indicated in the table, only 1.2 percent of all reported single-vehicle, ran-off-road accidents exceed the test conditions of 100 km/h and 25 degrees. If the impact angle is reduced to 20 degrees, the percentage of crashes exceeding the test conditions would increase to 2 percent, which is still only a small fraction of reported accidents.

 

TABLE 3. RELATIVE KINETIC ENERGY FOR SELECTED IMPACT CONDITIONS

Speed (km/h)

Angle (deg)

% Exceeding Both

100

25

1.2

100

20

2.0

110

20

0.9

120

20

0.4

If the impact speed is increased to 110 km/h (68.4 mph) and the angle is reduced to 20 degrees, the percentage of accidents exceeding the test conditions would actually be reduced slightly to 0.9 percent. The percentage drops further when the impact speed is further increased to 120 km/h (74.6 mph). However, it should be noted that this accident data was collected during the time when the national speed limit was 88.5 km/h (55 mph). It is likely that with the higher speed limits which presently exist, the impact speed distribution will have increased while the impact angle will have decreased. Unfortunately, there is no data available to determine if and how much the distributions of impact conditions have changed as a result of higher speed limits.

Lateral Offset Relationship

There are some physical relationships that exist between speed and angle that can be explored to provide further insight into this issue. Figure 1 shows the maximum attainable angle that can be attained in a single steering maneuver for a given speed and lateral offset based on a point-mass model. The specific relationship shown in Figure 1 assumes maximum steering and a coefficient of friction between the tires and pavement of 0.7.

This model indicates that in order to achieve an impact angle of 25 degrees at a speed of 100 km/h, the lateral offset from the barrier at the onset of the steering maneuver must be about 10.5 m (34 ft) for the conditions assumed in the analysis. This lateral offset is well beyond the typical offset for a barrier and is even beyond the required clear zone width for many roadways.

Further examination of this model shows that, for a given lateral offset, the maximum attainable angle decreases as the speed increases. The required offsets for selected speed and angle combinations using this model are shown in Table 4. For example, at a speed of 100 km/h, the lateral offset distance required to attain an impact angle of 20 degrees is reduced to approximately 6.7 m (22 ft), a distance which roughly corresponds to the distance an errant vehicle would travel to impact a barrier on the left side (or opposite side) of a 2-lane roadway with a shoulder. At a speed of 110 km/h (68.4 mph), the corresponding lateral offset required to reach an angle of 20 degrees is 8.2 m (27 ft).

TABLE 4. REQUIRED OFFSETS FOR SELECTED SPEED AND ANGLE COMBINATIONS

Speed (km/h)

Angle (deg)

Required Offset (m)

100

25

10.5

100

20

6.7

110

208.2
110 25 12.5

This same model can be used to investigate the speed and angle relationship for lower test levels as well. Currently, the only difference between Test Levels 1, 2, and 3 is the impact speed which has values of 50, 70, and 100 km/h, respectively. Since the maximum attainable impact angle increases with a decrease in speed, it may be appropriate to vary both the speed and the angle when defining the impact conditions for each test level.

However, it should be noted that for accidents in which multiple impacts and/or steering maneuvers occur, much greater impact angles than those indicated by the point-mass model can occur. This should be kept in mind when considering the relevance of this model for the selection of impact speed and angle combinations.

TEST MATRICES

Considerable revisions to the crash test matrices were incorporated into NCHRP Report 350. In addition to adoption of a multiple test level concept, changes included the addition of test matrices for new features such as work zone traffic control devices and truck-mounted attenuators. Furthermore, additional test requirements were added to the test matrices for other devices. Most notably, the matrix for terminals and crash cushions was expanded from four to eight tests. Various optional tests were also included in Report 350 and encouraged to be run. These included tests with a 700-kg small car and some offset impacts with TMAs.

While all of these test matrices were well though out with supporting rationale for the changes, various issues have been raised in regard to some of these tests as experience has been gained with Report 350 crash testing. This section of the paper will identify and briefly discuss some of the issues related to the test matrices contained in Report 350 for consideration in future updates.

Terminals/Crash Cushions (Test 34)

Test 34 is a small car redirection test for gating terminals and crash cushions. For TL-3, this test (i.e., Test 3-34) involves an 820 kg vehicle impacting at 100 km/h and 15 degrees. The purpose of this test is to evaluate occupant risk and vehicle trajectory criteria. The impact location is specified to be at the critical impact point (CIP) between the nose of the device and the beginning of length of need (LON). Although a default impact location is specified for terminals, no other guidance is given in terms of how to select the CIP for this test condition. This can possibly result in variability in testing among systems considered to be of a similar nature.

Terminals/Crash Cushions (Test 39)

Test 39 is a reverse direction impact for guardrail terminals and crash cushions. It involves a 2000-kg pickup truck impacting at 20 degrees. This test was incorporated into Report 350 to evaluate the potential for snagging when a device is impacted by traffic traveling in the direction opposite to its intended use. The impact point is specified to be L/2 where L is the overall length of the terminal.

Several issues have been raised regarding this test, the most basic of which is whether or not this test is necessary. Results of crash tests suggest this test is rather benign for many guardrail terminals and crash cushion designs. It may be appropriate to consider making this test applicable only to those devices which are judged to pose a snagging hazard for the reverse direction.

Another issue pertains to the appropriateness of the 2000P as the design test vehicle. Depending on the system being evaluated it can be argued that the 2000P is not the most critical vehicle. For example, the 820C may be more critical for guardrail terminals using a cable anchor assembly due to its increased propensity for underriding the rail. On the other hand, the 2000P may be more critical for crash cushions which use overlapping fender panels to provide redirectional capability.

A third issue regarding this test condition is impact location. For systems with a cable anchor assembly, such as W-beam guardrail terminals, the CIP may be closer to the nose of the terminal than the L/2 point. Also, the impact point can vary depending on the definition or interpretation of the length of the terminal, and perhaps a more precise definition should be considered.

Terminals/Crash Cushions (Tests 32 and 33)

Tests 32 and 33 are 15 degree angle impacts on the nose of a terminal or crash cushion with the 820C and 2000P test vehicles respectively. These tests are intended to evaluate occupant risk and vehicle trajectory criteria. They are currently considered optional for "gating" terminals such as the MELT and SRT.

Discussions regarding these test conditions have focused on what constitutes a "gating" terminal and whether these devices should be exempt from these tests. Another issue is whether or not both of these tests are necessary. The test with the 820C vehicle is generally considered to be more critical than that with the 2000P and results of crash tests seem to support this position. Consequently, it may be appropriate to eliminate Test 33 with the pickup truck.

Terminals/Crash Cushions (Test 30)

Test 30 is a small car, end-on impact for terminals and crash cushions. In this test configuration, the vehicle is offset such that the 1/4 point of the vehicle impacts the centerline of the nose of the device. This test is intended to evaluate occupant risk and vehicle stability.

This impact configuration seems to be critical for flared terminals because the eccentricity created by the offset impact makes the vehicle yaw or rotate, thereby increasing the propensity for rollover and occupant compartment intrusion of hardpoints through the exposed side of the vehicle.

However, for tangent, energy-absorbing devices, this offset configuration may not be as critical as a centerline impact. Although the induced rotation or yaw makes this impact configuration more critical from the standpoint of vehicle stability, it results in less energy to manage. Therefore, if the crush characteristics of a device is optimized based on this offset condition, it may not safely accommodate centered impacts with small cars which, for the same impact speed, require more ridedown distance. It may therefore be appropriate to require an additional centered, end-on test for tangent devices in addition to the 1/4 point offset test.

Terminals/Crash Cushions (Transition)

An area which is somewhat unclear in Report 350 is the transition of a terminal or crash cushion to a standard barrier section. Many terminals have weakened posts and/or a flared geometry which increases the potential for snagging or pocketing on the standard barrier system. An additional test may be needed in the terminal/crash cushion matrix to be properly evaluate the structural adequacy of this transition region.

However, given that such a test is desirable, what are the appropriate impact conditions. The standard transition test for longitudinal barriers (Test 21) is conducted at 25 degrees. But the strength test for terminals, conducted at the beginning of the terminal LON, uses an impact angle of 20 degrees. Since the transition from the terminal to standard barrier section is typically considered part of the terminal, it may be appropriate to use a 20-degree impact angle rather than 25 degrees.

Truck Mounted Attenuators (Tests 52 & 53)

Tests 52 & 53 are offset impacts with the 2000P which are intended to be part of the evaluation of truck mounted attenuators (TMAs). However, these tests were made optional since there was no experience with these impact conditions and it was uncertain if current TMA designs could meet these requirements without major modifications or increases in cost. Not surprisingly, there has been little, if any, testing conducted with these optional tests.

While these tests are believed to be representative of many collisions with TMAs, their relevance to real-world crashes has yet to be established. In order to make these test meaningful, it may be appropriate to investigate the relevance of these impact conditions and decide whether these tests should be required or removed.

Truck Mounted Attenuators (Test 3-50)

Test 3-50 is a small car, head-on test into a TMA at 100 km/h. The test is intended to evaluate occupant risk. Report 350 recommends that the support truck to which the TMA is attached be placed against a rigid support structure to prevent any forward movement. Based on manufacturers concerns that current TMA technology could not meet these requirements for high speed impacts without major modifications or increases in cost, FHWA modified the support truck configuration for this test. The revised configuration is similar to Test 51 with the 2000P which requires the support truck to be placed in 2nd gear with the brakes set.

Recently, some TMA designs have successfully passed Test 3-50 with the support truck placed against a rigid wall. Future updates to Report 350 should examine the relevance of this issue in order to determine which impact configuration is more appropriate.

Optional Tests

The test matrices in Report 350 include several optional tests. Most of these optional tests pertain to testing with a 700-kg passenger car (700C) which is considered a supplementary test vehicle. The 700C test vehicle was included in recognition of the downsizing trend that was prevalent in the 1970s and 1980s. However, since the publication of Report 350, this trend has reversed itself and the weight and size of small passenger cars has been increasing. As an illustration, there is not a single vehicle model with a curb weight under 820 kg (1808 lb.) in the 1995 or 1996 vehicle fleet. Thus there appears to be no reason to retain these optional tests with the 700C vehicle in future updates of the crash testing guidelines.

Side Impact Testing

It has been estimated that 25 percent of single-vehicle, fixed object fatal crashes involve side impacts. Field performance and full-scale crash testing has shown that some roadside safety appurtenances such as crash cushions, terminals, and breakaway support structures may not perform satisfactorily under side or non-tracking impact conditions A recently Federal Highway Administration (FHWA) examined the side impact issue and recommended new side impact test procedures which are in included in an appendix of Report 350. FHWA has recently sponsored another study to conduct further investigation into this issue. The results of this study should be critically reviewed and considered for incorporation into future updates of Report 350.

SUMMARY

Several issues related to impact conditions and test matrices have been identified for consideration in future updates of the crash test and evaluation procedures presently recommended in NCHRP Report 350. It should be noted that many of these issues need to be addressed in conjunction with other topic areas such as test vehicles and evaluation criteria. Furthermore, the relevance of each issue, in terms of the associated benefits and costs, should be addressed to the extent possible given the limitations of existing data.


OTHER SAFETY PERFORMANCE EVALUATION CRITERIA
Maurice E. Bronstad
Dynatech Engineering, Inc.

INTRODUCTION

With the publication of NCHRP Report 230 in 1981, 1 an objective occupant risk criteria with processed vehicle acceleration values directly measured during crash test experiments became the primary evaluation criteria for pass-fail judgment. The current NCHRP Report 350 2 retained essentially the same criteria in metric format.

Other evaluation criteria were included in these documents and will be the focus of this paper. There is some question about the relative importance and the priority for these many other criteria.

OTHER EVALUATION CRITERIA

Tables 6 and 5.1 from Reports 230 and 350 respectively are shown in Tables 1 and 2. These criteria will be discussed in following sections.

Structural Adequacy

These criteria basically describe desired behavior for longitudinal barriers, breakaway/yielding supports, and terminals/crash cushions. Consideration of debris from tests was in this category in Report 230, but was included in occupant risk category of Report 350.

Redirection Criteria: These criteria basically require longitudinal barriers, redirective crash cushions and terminals at and beyond the length-of-need (LON) to contain and redirect vehicles under the conditions of impact for the designated Test Level (TL). Some judgment is required, but if other factors are met, this is usually not a determining pass-fail criteria. As defined in Report 350 glossary, undesirable behavior such as pocketing or snagging can result in unacceptable vehicle acceleration values. Some "gray" examples are shown in Figure 1. In Figure 1(a), a dip in edge of approach terrain was determined to be cause of the underride behavior, however, the acceleration value criteria of Report 350 (i.e., occupant velocity of 12 m/s) was not exceeded. The exit angle did exceed the 60 percent which is discussed in a succeeding section (B.3.B). In Figure 1(b), one strand of a cable guardrail system overrode the vehicle at one point and interacted with the A-pillar of the vehicle. As shown in the at-rest photograph, the top strand did eventually end up under the hood. Operating under NCHRP Report 230, the test of 1(a) was deemed a failure whereas the 1(b) test was judged to be successful.

QUESTION: Could these criteria be combined with the redirection criteria in Vehicle Trajectory (B.3.B in this paper)? Does "should not penetrate" mean no separation of a metal beam or controlled fracturing of a concrete beam? What do we mean by underride and override? Can this be quantified, and how important is it?

Breakaway/Yielding: A device designed to breakaway/yield under the desired TL must perform in this manner during the crash test. Failure to breakaway or yielding in soil are good reasons to reject a design. Figure 2 contains some examples of failed designs.

QUESTION: Is it clear that devices that do not perform as designed in crash tests should be "failed" even if they pass all other evaluation criteria?

Design Principles: These criteria describe the different principles of design for a crash cushion and terminal. The terms gating and non-gating were introduced in Report 350. Basically, the design principles of some devices achieve redirection under TL conditions very near the nose or beginning of the device as shown in Figure 3b, this defines a non-gating device. Another device may have the LON at some defined distance from the nose as shown in Figure 3b, this describes a gating device. Generally, non-gating devices will be more expensive than gating devices. The location of the LON is part of the layout procedure for these devices, as illustrated in Figure 4 from the 1996 AASHTO, Roadside Design Guide 3, and either are acceptable as long as layout procedures are properly followed. The designer must be aware of the consequences of the vehicle barely missing a non-gating device and proceeding into the same area at higher speed than a vehicle passing through a gating device as shown in Figure 4.

QUESTION: Do we need gating and non-gating categories?

Occupant Risk

The format of Report 350 includes four evaluation sections excluding those directly related to measured vehicle acceleration values.

Detached Debris/Fragments: These criteria can become very subjective because of the use of the word "potential", and the myriad of possibilities for fragment dispersion during high-speed vehicle impacts with devices ranging from breakaway posts/supports to lightweight construction barricades. The hazards of this debris can be inflicted on following or adjacent traffic, pedestrians and construction workers, or to the occupants of the impacting vehicle.

QUESTION: Can we prepare a reporting format and/or an evaluation criteria?

Following or Adjacent Traffic

Although not necessarily a cause for failure, the debris pattern and significance (size, mass, etc.) observed in full-scale crash tests should be recorded for consideration by a user agency. Figure 5 provides examples of these considerations.

Pedestrians and Construction Workers

The significance of debris for personnel should be judged as opposed to that for occupants in following or adjacent vehicles.

Debris Penetrating Into the Occupant Compartment

While it is undesirable for any debris to penetrate into the occupant compartment, some judgment is required regarding this subject. A good example is the performance of a common call box support structure. As shown in Figure 6a, during breakaway performance, local windshield and header deformation can occur, however, the velocity change of these collisions is so small, the occupant's head does not contact this area with appreciable velocity, if at all. As shown in Figure 6b, other tests with this support have resulted in local deformation of the rear window area, allowing some glass fragments to enter the rear passenger compartment. While this is not desirable, it is unclear how much hazard is represented by this. Injuries would not be life threatening and problematical.

QUESTION: Do we mean that any debris penetrating into passenger compartment is a cause for "failure?" Can we further define or quantify this?

Passenger Compartment Deformation

The passenger compartment space has been characterized for passenger vehicles as illustrated in Figure 7. The occupant risk acceleration based criteria are based on an undeformed passenger compartment. If changes in the head-related space as illustrated in Figure 5a are experienced, the deformed or actual values should be used.

Relationship to other parts of the occupant's body become more complicated. Leg/lower torso injuries related to floor pan deformation are not well understood. Dash and steering column movement toward the occupant can inflict additional injury, but there is no mechanism for relating this complication. Figure 7 represents data that are routinely recorded for each vehicle crash-tested by NHTSA. It is recommended that an effort be made to obtain these data from NHTSA for future reference.

Use of lighter weight construction and materials as well as pickups with more open engine compartments have resulted in more passenger compartment deformation in recent crash tests. While NCHRP Report 350 presents a method for measuring passenger compartment deformation, there are no pass-fail values for this consideration.

Figure 8 contains examples of passenger compartment deformations.

QUESTION: Can we develop a measurable pass-fail criteria for passenger compartment deformation?

Detached Elements Blocking or Causing Blocking of Driver's Vision: This is a very subjective criteria basically related to construction zone devices resulting in minimal velocity change which causes relatively long-term vision blockage while the driver regains vehicle control. An example of this behavior is shown in Figure 9.

QUESTION: Would this consideration ever be used to disqualify a product?

Vehicle Angular Stability: The roll, pitch, and yaw behavior of the vehicle is indicative of vehicle "stability" and "test repeatability." Large roll and pitch excursions can result in or be a precursor to vehicle rollover/tumbling. Excessive yawing of passenger vehicles on pavement can be benign, but in softer soil can result in tripping of the vehicle. Higher c.g. vehicles can roll on the pavement, whereas, most passenger vehicles will slide before friction sufficient for tripping can be developed. It is not clear how low energy rollovers of passenger vehicles with resultant 1/4 roll should be judged. An example is shown in Figure 10. We know from analytical evidence that limbs can be lost and unrestrained passengers ejected in 1/4 roll.

The rollover criteria in Report 350 is basically based on the art-of-the-possible. It is "possible" to keep the 820-C and 2000-P vehicles upright, but for tests with higher c.g. vehicles (i.e., 8000 S, 36,000 V, and 36,000 T) rollover is not a cause for failure due to the difficulty in achieving upright conditions in the experiments. There have been some observed rollover problems with current hardware with the 2000-P pickup. It is generally believed that it is not cost-effective to provide hardware to manage the large energies and inherent instability of large vehicles.

Much insight into the large truck fatality problem is gained from a recent UMTRI report 4. About 20 percent of the fatalities were in single vehicle accidents. About 60 percent of all truck driver fatalities involve the truck rolling over. Although reported restraint usage is increasing among truck drivers, only 20 percent of the fatally injured drivers were restrained in 1994. Figure 11 provides some data from this study.

QUESTION: Can we quantify excessive roll, pitch, and yaw values? Can we accept a 1/4 roll in pickup and other higher c.g. vehicle tests?

Vehicle Trajectory

These criteria are related to the hazard represented by the striking vehicle to following or adjacent traffic or to other considerations.

Striking Vehicle Trajectory Into Adjacent Lanes This criteria is not used in evaluating devices on pass-fail basis. There are applications of barriers with minimal shoulders where the vehicle would never leave the lane. Repeatability of tests would no doubt render this criteria invalid. Examples are shown in Figure 12.

QUESTION: Should this statement be confined to text?

Redirection During Structural Adequacy Tests. This criteria is measurable, but we believe that it has never been applied to fail a test. For example, the test shown in Figure 13 passed this criteria.

QUESTION: Should redirection criteria in structural adequacy test be retained?

Exit Angle. The magnitude of this 60% angle is totally inconsistent with B.3.A and is much less demanding. This is a surrogate for the hazard imposed to following traffic or for involvement with other potential hazards. Figure 14a is presented for a discussion of this. It is noteworthy that vehicle heading angles at the instant of exiting the barrier can change dramatically as illustrated in Figure 14.

QUESTION: IS this a valid criterion?

Vehicle Trajectory Behind Test Article. This criteria demonstrates the insignificance of gating/non-gating devices. Trajectory behind the non-gating device is permitted in four of the tests and five of the gating tests as shown in Figures 3 and 4.

QUESTION: Should we retain gating/non-gating categories?

CONCLUSIONS

Many of the evaluation criteria in Report 350 require some judgment on a pass-fail basis. Other observed behavior should become part of the application of the device (e.g., debris fragments may limit a terminal to shoulders or wide medians).

Measurable values that can be compared to acceptable values provide the most sound and even-handed evaluation criteria. These acceptable values should have as strong a basis as possible to achieve the injury-producing goals of roadside safety.

Passenger compartment deformation could become a more significant consideration. It may be necessary to provide more meaningful objective criteria for this consideration.

Subjective judgment failures of a device can be contentious, and every effort should be made to provide measurable objective criteria. Obviously, we are not at this stage in our technology for all the discussed factors, but it is a worthy goal. As an example there are many in our community that believe that for redirective devices, vehicles that are smoothly redirected without severe snagging or pocketing, and remain upright will provide occupants with minimal injury causation.

REFERENCES

1. J.D. Michie, "Recommended Procedures for the Safety Performance Evaluation of Highway Appurtenances," NCHRP Report 230, 1981.

2. H.E. Ross, et al, "Recommended Procedure for the Safety Evaluation of Highway Features," NCHRP Report 350, 1993.

3. 1996 AASHTO "Roadside Design Guide".

4. UMTRI "Research Review," Univ. of Michigan Transportation Research Institute, October- November 1996.

LIST OF TABLES

TABLE 1 NCHRP REPORT 230 EVALUATION CRITERIA

Evaluation Factors

Evaluation Criteria

Applicable to Minimum Matrix Test Conditions (see Table 3)

Structural Adequacy

 

 

 

 

 

 

 

A. Test article shall smoothly redirect the vehicle; the vehicle shall not penetrate or go over the installation although controlled lateral deflection of the test article is acceptable.

B. The test article shall readily a predictable manner by breaking away or yielding.

C. Acceptable test article performance may be by redirec-tion, controlled penetration, or controlled stopping of the vehicle.

D. Detached elements, fragments or other debris from the test article shall not penetrate or show potential for penetrating the passenger compartment or present undue hazard to other traffic.

10, 11, 12, 30, 40

 

 

60, 61, 62, 63

41, 42, 43, 44, 45, 50, 51, 52, 53, 54

All

Occupant Risk

E. The vehicle shall remain upright during and after collision although moderate roll, pitching and yawing are acceptable. Integrity of the passenger compartment must be maintained with essentially no deformation or intrusion.

F. Impact velocity of hypothetical front seat passenger against vehicle interior, calculated from vehicle accelerations and 24 in. (0.61m) forward and 12 in. (0.30m) lateral displacements, shall be less than:

Occupant Impact Velocity-fps

Longitudinal Lateral

40/F1 30/F2

And vehicle highest 10 ms average accelerations subsequent to instant of hypothethical passenger impact should be less than:

Occupant Ridedown Accelerations - g’s

Longitudinal Lateral

20/F3 20/F4

where F1, F2, F3, and F4 are appropriate acceptance factors (see Table 8, Chapter 4 for suggested values).

G. (Supplementary) Anthropometric dummy responses should be less than those specified by FMVSS 208, i.e., resultant chest acceleration of 60g, Head Injury Criteria of 1000, and femur force of 2250 lb (10kN) and by FMVSS 214, i.e., resultant chest acceleration of 60g, Head Injury Criteria of 1000 and occupant lateral impact velocity of 30 fps (9.1 m/s).

All

 

 

 

 

 

11, 12, 41, 42, 43, 44, 45, 50, 51, 52, 54, 60, 61. 62, 63

 

 

 

 

 

 

 

11, 12, 41, 42, 43, 44, 45, 50, 51, 52, 54, 60, 61, 62, 63

Vehicle Trajectory

H. After collision, the vehicle trajectory and final stopping position shall intrude a minimum distance, if at all, into adjacent traffic lanes.

I. In test where the vehicle is judged to be redirected into or stopped while in adjacent traffic lanes, vehicle speed change during test article collision should be less than 15 mph and the exit angle from the test article should be less than 60 percent of test impact angle, both measured at time of vehicle loss of contact with test device.

J. Vehicle trajectory behind the test article is acceptable.

All

 

10, 11, 12, 30, 40, 42, 44, 53

 

 

41, 42. 43, 44, 45, 50, 51, 53, 54, 60, 61, 62, 63

 

TABLE 2 NCHRP REPORT 350 EVALUATION CRITERIA

Evaluation Factors

Evaluation Criteria

Applicable Testsa

Structural Adequacy

A. Test article should contain and redirect the vehicle; the vehicle should not penetrate, underride, or override the installation although controlled lateral deflection of the test article is acceptable.

B. The test article should readily activate in a predictable manner by breaking away, fracturing, or yielding.

C. Acceptable test article performance may be by redirection, controlled penetration, or controlled stopping of the vehicle.

10, 11, 12, 20, 21, 22, 35, 36, 37, 38

 

60, 61, 70, 71, 80, 81

30, 31, 32, 33, 34, 39, 40, 41, 42, 43, 44, 50, 51, 52, 53

 

Occupant Risk

 

 

 

 

 

 

 

 

 

 

D. Detached elements, fragments or other debris from the test article should not penetrate or show potential for penetrating the occupant compartment, or present an undue hazard to other traffic, pedestrians, or personnel in a work zone. Deformations of, or intrusions into, the occupant compartment that could cause serious injuries should not be permitted. See discussion in Section 5.3 and Appendix E.

E. Detached elements, fragments or other debris from the test article, or vehicular damage should not block the driver’s vision or otherwise cause the driver to lose control of the vehicle.

F. The vehicle should remain upright during and after collision although moderate roll, pitching and yawing are acceptable.

G. It is preferable, although not essential. That the vehicle remain upright during and after collision.

 

 

 

All

 

 

 

 

 

70, 71

 

 

All except those linked in Criterion G.

12, 22, 30b, 31b, 32b, 33b, 34b, 35b, 36b, 37b, 38b, 39b, 40b, 41b, 42b, 43b, 44b

 

 

 

H. Occupant impact velocities (see Appendix A, Section A5.3 for calculation procedure) should satisfy the following:

 

 

Occupant Impact Velocity Limits (m/s)

Component

Preferred

Maximum

 

Longitudinal and Latitudinal

 

9

 

12

10, 20, 30, 31, 32, 33, 34, 36, 40, 41, 42, 43, 50, 501, 52, 53, 80, 81

Longitudinal

3

5

60, 61, 70, 71

Occupant Risk

I. Occupant ridedown accelerations (see Appendix A, Section A5.3 for calculation procedure) should satisfy the following:

 

 

Occupant Ridedown Acceleration Limits (G’s)

Component

Preferred

Maximum

 

Longitudinal and Latitudinal

 

15

 

20

10, 20, 30, 31, 32, 33, 34, 36, 40, 41, 42, 43, 50, 51, 52, 53, 60, 61, 70, 71, 80, 81

J. (Optional) Hybrid III dummy. Response should conform to evaluation criteria of Part 571.208, Title 49 of Code of Federal Regulation, Chapter V (10-1-88 Edition). See Section 5.3 for limitations of Hybrid III dummy.

10, 20, 30, 31, 32, 33, 34, 36, 40, 41, 42, 43, 50, 51, 52, 53, 60, 61, 70, 71, 80, 81

 

Vehicle Trajectory

K. After collision it is preferable that the vehicle’s trajectory not intrude into adjacent traffic lanes.

All

L. The occupant impact velocity in the longitudinal direction should not exceed 12 m/sec and the occpuant ridedown acceleration in the longitudinal direction (see Appendix A, Section A5.3 for calculation procedure) should not exceed 20 G’s.

11, 21, 35, 37, 38, 39

M. The exit angle from the test article preferably should be less than 60 percent of test impact angle, measured at time of vehicle loss of contact with test device.

10, 11, 12, 20, 21, 22, 35, 36, 37, 38, 39

N. Vehicle trajectory behind the test article is acceptable.

30, 31, 32, 33, 34, 39, 42, 43, 44, 60, 61, 70, 71, 80, 81

aTest numbers refer to last two digits in Test Designation for each Test Level unless otherwise noted.

bFor Test Level 1 only.

 

LIST OF FIGURES

FIGURE 1(a) Vehicle underride of three beam - w-beam transition.

FIGURE 1(b) G1 cable guardrail crash test.

FIGURE 2 Examples of poor performance of breakaway/yielding supports.

FIGURE 3 Gating/non-gating considerations(2).

FIGURE 4 Gating/non-gating considerations.

FIGURE 5 Debris after crash tests.

FIGURE 6(a) 20 mph call box support crash test.

FIGURE 6(b) Two other call box support crash tests.

FIGURE 7 NHTSA passenger compartment measurements from crash tests.

Figure 7(b) Dummy Measurement Data For Front Seat Occupants.

FIGURE 8 Passenger compartment deformation measurement.

FIGURE 9 Barricade crash test resulting in driver vision problem.

FIGURE 10 Test of G9 guardrail with 8000-lb. van at 60 mph and 25 deg.

FIGURE 11 UMTRI fatal truck accident data (4).

FIGURE 12 MB3 box beam median barrier tests using 1800-lb. cars at 60 mph.

FIGURE 13 Unsatisfactory test results, but the longitudinal occupant risk criteria values were not exceeded.

FIGURE 14(a) Occupant risk test of MB3 (20-deg angle).

FIGURE 14(b) Occupant risk test of MB3 (20-deg angle) continued.


 

ALTERNATIVE TECHNIQUES FOR EVALUATING ROADSIDE SAFETY SYSTEMS
Dean Sicking
University of Nebraska

The performance of roadside safety devices has traditionally been evaluated through full-scale crash testing. Although full-scale crash testing has proven to be an effective method of evaluating the impact performance of roadside safety systems, the associated high costs of research have severely limited development of new systems. A number of alternative evaluation procedures have been developed in an effort to reduce costs. The alternate procedures include using scale models, surrogate vehicle testing, and computer simulation. Each of these techniques is described below and examples showing successful application are included.

Scale Models

Scale modeling involves building and testing small-scale models of a device to estimate the response of a full-scale system subjected to similar conditions. Forces and accelerations measured during these tests must be scaled up to evaluate the behavior of full-scale systems. Developing the appropriate scaling factor is the greatest obstacle that must be overcome in order to make practical use of scale model testing. This process involves first identifying all system characteristics that control forces on the device to be studied. As an example, consider scale model tests of a cylindrical rubber energy absorber. The energy absorbed by a rubber cylinder is controlled by the element diameter, wall thickness, length, and the rubber's modulus of elasticity. The scaling factor relating model tests to full-scale prototype performance was found to be:

where:

D = outer diameter

d = inner diameter

L = length

Eo = initial Modulus of Elasticity

Scale model testing of rubber cylinders proved to be a very valuable tool for designing crash cushion energy absorbing elements.

Scale modeling can be used effectively for this and many other design problems where the number of design parameters is relatively limited and vehicle characteristics are not a consideration. However, when vehicle characteristics are important or when the number of important design parameters becomes increases, scale modeling becomes very difficult and as a result very costly. Consider for example, scale model testing to evaluate guardrail design. A list of factors that can have an important effect on guardrail performance during a vehicular impact are shown in Table 1. Developing scaling factors for such a large number of factors is functionally impossible. Therefore, scale model testing cannot be used as a substitute for compliance testing. However, as described previously, scale model testing remains a valuable tool for use in development of new safety devices.

TABLE 1. FACTORS AFFECTING GUARDRAIL FORCES
Guardrail Installation Guardrail and Vehicle Elements
Guardrail Height Moment of Inertia
Post Spacing Section Depth
Post/Rail Attachment Section Width
Installation Length Yield Stress
Type of Anchor Mod. of Elasticity
Soil Compaction Material Toughness
Soil Angle of Internal Friction Splice/Connection Details
Soil Moisture Content  
 
Post Characteristics Overall Vehicle
Embedment Depth Weight
Moment of Inertia Length
Section Depth Width
Section Width Front Overhang
Mod. of Rupture Mass Moments of Inertia
Mod. of Elasticity C.G. Location
Size and Shape of Wood Knots  

Pendulum/Bogie Testing

Surrogate vehicle testing involves impacting a safety device with a pendulum or bogie vehicle. Pendulums and bogie vehicles incorporate a rigid frame with a crushable nose as shown in Figure 1. The crushable nose is constructed from honeycomb aluminum and is designed to replicate the front stiffness of a small car. As shown in Figure 1, the crushable nose systems are designed only for frontal impacts with relatively narrow objects, such as breakaway sign and luminaire supports. Thus, these vehicles cannot be used to measure the safety performance of longitudinal barriers and other safety devices subjected to oblique impacts. However, a serious question still remains regarding the appropriateness of using surrogate vehicles to test breakaway structures.

The crushable nose system currently used on breakaway devices was developed to replicate a 1981 Volkswagen Rabbit. The design of mini-size automobiles has changed dramatically during the last 16 years, especially with regard to energy management in the frontal structure. Whereas the Volkswagen Rabbit had a steel bumper and steel frontal structure, most modern mini-size cars are constructed with Styrofoam filled plastic bumpers and plastic frontal structures. The extensive use of plastic and Styrofoam makes these vehicles much softer than older designs.

Note that surrogate vehicles are most commonly used to test breakaway devices. These devices are designed to remain essentially rigid during the initial stages of impact. As the front of the impacting vehicle is crushed, forces on the breakaway mechanism increase. When impact forces reach sufficient levels to activate the breakaway mechanism, the structural support is knocked out of the path of the vehicle.

The performance of a breakaway device is normally measured in terms of velocity change during the impact. Vehicle crush is the primary source of energy dissipation during these impacts and has a strong effect on the velocity change during such a test. If the vehicle crush force versus depth of penetration is fairly linear, the vehicle crush energy can be expressed in terms of the activation force for the breakaway mechanism:

E = F2
      K

Where:

E = energy dissipated by the vehicle structure
F = activation force
K = vehicle frontal stiffness

As shown in this equation, vehicle energy dissipation is inversely proportional to vehicle stiffness. Soft vehicles have much higher crush energy dissipation than do stiff vehicles. Thus, a softer frontal structure would have a much higher velocity change during an impact with a breakaway device than a vehicle with a stiffer frontal structure. Therefore it is imperative that if surrogate vehicles are to be used to evaluate the performance of breakaway structures, the frontal crush stiffness needs to accurately reflect the design of modern small cars. It is unacceptable to use the current crushable nose which replicates a 16 year old vehicle.

Although surrogate vehicle testing can accurately predict vehicle velocity change if an appropriate frontal crush stiffness is used, there are still other concerns that should be addressed. Many breakaway systems, such as breakaway luminaries, utility poles, and call boxes, have the propensity for falling on top of impacting vehicles. Serious injuries have been reported when heavy supports crush the top of a vehicle occupant compartment. Since bogies and pendulums do not have a roof structure, it is impossible to evaluate the seriousness of a structural support falling on to one of these vehicles. Therefore, when a structural support falls onto a surrogate vehicle, a full-scale crash test must be conducted to accurately assess the safety of the breakaway device.

Another significant shortcoming of surrogate vehicles is the absence of a true suspension. Some full-scale tests of breakaway devices have resulted in vehicles rolling over due to snagging on the post stub or inappropriate bending of the structural support. These rollovers cannot be accurately replicated by either a pendulum or most bogie vehicles in use today. Pendulums are suspended from a structure and therefore cannot rollover. Although bogie vehicles can be used to replicate high speed impacts, the suspensions used on most of these devices to not attempt to replicate an automobile. As a result, bogie vehicles cannot assess the propensity for rollover either.

Computer Simulation

Computer simulation of roadside safety devices involves using nonlinear finite element analysis programs to model ran-off-road accidents. There a three primary applications of this technology, component design, test extrapolation, and replacement of full-scale crash testing. As summarized below these alternatives are listed in order of increasing difficulty and cost.

Component design using computer simulation involves modeling critical elements of a roadside safety structure. One example of this technique was a recently completed study to develop a modified fuse plate design. A model of the fuse plate was developed as shown in Figure 2. LS-DYNA was then used to model the force deflection characteristics of the fuse plate under typical impact conditions. The computer simulation allowed researchers to model many combinations of materials, plate thicknesses, and perforation geometries to select an optimal design. The candidate design was then tested dynamically to verify simulation findings. After completion of the dynamic testing, full-scale crash tests were conducted to verify the overall performance of the new breakaway mechanism. In this application, computer simulation increased the number of design alternatives that could be evaluated and greatly reduced the cost of the evaluation for each alternative.

Development of the SKT-350 guardrail terminal is another example of using computer simulation in the design of roadside safety devices. The impact head for this energy absorbing guardrail terminal was designed using LS-DYNA. The simulation effort involved designing the deflector system to control the energy dissipation rate. For this simulation, a model of the deflector system being pushed down the guardrail was used to predict the force deflection characteristics of the new energy absorbing head. Deflector system geometry was modified until the energy absorption rate was tuned to a desirable level. Static testing was used to verify the energy dissipation rate prior to the start of full-scale crash testing. In this case, simulation replaced an extensive static test program and thereby greatly reduced the time and cost involved in the development effort.

Bogie testing was undertaken to verify the full-scale impact performance of the impact head and although results verified simulated energy dissipation rates, these tests also indicated that the structure of the impact head needed reinforcement. LS-DYNA was again used to redesign the impact head. In this case, a model of the impact head, shown in Figure 3, was subject to high speed impact with a model of a 3/4 ton pickup. The size and configurations of impact head reinforcing plates were then modified until the simulation indicated that no damage would develop during high speed impacts. Full-scale crash testing of the new terminal design was then undertaken with excellent results. Only six full-scale crash tests were required to meet NCHRP Report 350 safety performance standards. The computer simulation was instrumental in achieving the development goals at a greatly reduced cost compared to conventional design procedures using component and full-scale testing. Thus, computer simulation is a very valuable tool for designing components of roadside safety devices.

Another application of computer simulation is in the area of extrapolation of full-scale crash test findings. The high cost of full-scale crash tests greatly limits the number of tests that can be conducted to investigate the safety of any given device. Further, crash testing procedures are limited to tracking type of impacts, while accident data indicates that 50 percent of all ran-of-road accidents involve non-tracking vehicles. Finally, roadside safety devices are seldom installed in the field under the same ideal conditions used in testing. For example, roadside slopes placed in front or behind a safety device can greatly affect its safety performance. Thus, full-scale crash testing cannot explore all of the impact scenarios that may lead to injuries or fatalities in real worlds accidents. In these cases, computer simulation is the only available method for fully exploring the performance of a roadside geometry or a safety device.

One of the best examples of this application of computer simulation was a study of vehicle stability on roadside slopes. In this effort, a relatively small number of full-scale crash tests were conducted to explore vehicle stability on roadside slopes. These test results were then used to validate the HVOSM simulation model. This program was then used to explore the stability of a wide range of vehicles encountering many different slope configurations. Findings from this simulation effort were implemented to establish existing roadside slope design criteria.

As computer simulation tools, such as LS-DYNA, become more powerful, it is possible to explore the off-tracking impact performance of roadside safety hardware. Although no major study has been completed to date, a number of efforts are currently underway. In this case, questions about the validity of the simulation programs are less critical, since simulation is the only available method for examining these issues.

Using computer simulation as an alternative for full-scale crash testing is the ultimate dream of safety hardware developers. Extensive validation of the computer simulation tools must be completed, carefully analyzed, and widely accepted before this dream can be realized. Unfortunately, the computer simulation community is still struggling with the question of what constitutes acceptable validation. Thus, using computer simulation as an alternate to full-scale crash testing is still out of reach. Nevertheless, as researchers gain skill, simulation codes are refined, and the speed of computer hardware increases, the dream of using this technology to reduce the need for full-scale crash testing begins to appear feasible.

Summary and Conclusion

Three alternatives to full-scale crash testing, scale model testing, surrogate vehicle vehicle testing, and computer simulation have been described. Scale model testing has proven to be a valuable tool in the design of specific components for roadside safety devices. However, developing accurate scale models of a vehicle is too costly to ever offer a practical alternative to full-scale crash tests.

Surrogate vehicle testing, involving pendulum and bogie vehicles, has been accepted as an alternative to full-scale testing of breakaway devices. Unfortunately, the implementation of these surrogate vehicle tests still has some major short comings. The crushable nose system used on all surrogate vehicles is out of date and must be revised to more accurately reflect the structure of modern vehicles. Further, since surrogate vehicles do not have a roof structure, the potential for occupant compartment intrusion cannot be evaluated. When surrogate vehicle testing indicates that a breakaway device will fall on top of an impacting vehicle, full-scale testing is still the only method for verify the system's safety performance. Finally, since bogie vehicles do not have accurate suspension models and pendulums are limited to low speed impacts, there is a concern that surrogate vehicle testing cannot accurately measure the potential for vehicle rollover during high speed impacts.

Computer simulation is now a very valuable tool for design roadside safety devices. Further, it is the only available method for examining many types of impacts that cannot be tested and it can be a valuable tool for extrapolating crash test results to other vehicles or impact conditions. Although computer simulation cannot now be used as an alternative to full-scale crash testing, technological advancements are bringing this alternative within the foreseeable future.


 

 

EVALUATION OF ROADSIDE FEATURES TO ACCOMMODATE VANS,
MINI-VANS, PICKUP TRUCKS AND 4-WHEEL DRIVE VEHICLES
Hayes E. Ross, Jr.
Texas Transportation Institute
Texas A&M University

NCHRP Project 22-11 began in June, 1994 and is scheduled to be completed in July, 1998. Project objectives are:

  • To evaluate current information on the safety performance of roadside features for each subclass of light trucks,

  • To assess the significance of gaps in safety performance information, and

  • To recommend priorities for future research, testing, and development needed to ensure that roadside features accommodate light trucks.

The project originated due to the recognition of the significant increases in the use of light trucks on US highways, and the mandates of the Intermodal Surface Transportation Efficiency Act (ISTEA) enacted in 1991 by the US Congress. The ISTEA instructed transportation agencies to design roadside features to accommodate light trucks.

The project is structured to undertake three basic efforts, namely, (1) collection of sales, crash test, and accident data for the light truck class of vehicles, (2) evaluate the effectiveness of the 2000P test vehicles, and (3) examine the impact performance of roadside features and light truck class vehicles through computer simulation, accident analysis, and crash testing. In the initial phase an effort was made to identify and acquire information on the light truck subclasses, including dimensional and inertial properties, sales and sales projections, accident data, and crash test data. A modest simulation study was conducted in the initial phase of the study to examine the performance of light trucks when impacting roadside barriers or when traversing roadside geometric features. Comparisons were also made between the properties of the larger light truck subclasses and the 2000P test vehicle, which is a 3/4-ton pickup truck. Results of the initial phase of the project were presented in an interim report.

A summary of the tentative findings of the project to date are as follows:

  • Light truck sales now comprise approximately 40 percent of all passenger vehicle sales in the US. In this project seven subclasses are used to categorize light trucks: mini-vans, large vans (1/2-ton and 3/4-ton), small pickup trucks, full-size pickup trucks (1/2-ton and 3/4-ton), small utility vehicles, mid-size utility vehicles, and large utility vehicles. Light trucks should obviously be a major factor in the design of highway safety features.

  • In comparison with the larger light trucks, the 2000P test vehicle appears to be a good, representative design vehicle. Key vehicular properties that influence the impact performance of a safety feature include bumper height, front overhang, total mass, the center of mass location, and mass moments of inertia. Sales of the large pickup truck subclass is also greater than any of the other light truck subclasses.

    · A number of crash tests have been conducted with the 2000P vehicle for test level 3 of NCHRP Report 350. Tests of the widely used median barriers and roadside barriers have been conducted. Most guardrail end treatments and crash cushions have been tested with the 2000P vehicle also. The following features have met test level 3 requirements:

Rigid barriers

  • New Jersey safety shaped concrete barrier

  • Single slope concrete barrier

  • Various bridge rails of state DOTs

Flexible barriers

  • G1 steel cable guardrail system

  • steel box beam guardrail system

  • G4(2W) W-beam guardrail system

Guardrail end treatments

  • ET-2000 (proprietary system)

  • BEST (proprietary system)

  • Slotted Rail Terminal (proprietary system)

Crash cushions

  • REACT 350 (proprietary system)

  • ADIEM (proprietary system)

  • QUADGUARD (proprietary system)

  • Sand-filled plastic drums

Systems that have failed test level 3 requirements include the following:

Flexible barriers

  • G2 W-beam guardrail system

  • G4(1S) W-beam guardrail system

  • G9 Thrie beam guardrail system

Marginal performance was observed in limited testing of segmental, precast concrete barriers having the New Jersey shape. Further testing is needed to evaluate the effects of segment length and joint design on impact performance.

The Highway-Vehicle-Object-Simulation-Model (HVOSM) and the BARRIER VII computer programs have been used to study the behavior of light trucks traversing roadside geometric features (embankments and driveways) and the behavior of light trucks when impacting longitudinal barriers. Results of these studies indicate that the smaller light truck subclasses have a greater propensity for overturning than the larger subclasses of light trucks.

Analysis of the FARS data for the 1991-95 period was made and a summary of the findings are as presented. These findings show that (1) light trucks are more prone to overturn in fatal crashes that cars, (2) there is a high incidence of overturn for all passenger vehicles in fatal crashes involving longitudinal barriers, (3) light trucks are more prone to overturn than cars in fatal crashes involving longitudinal barriers, and (4) there is a high incidence of overturn for all passenger vehicles in fatal crashes involving ditches and embankments.

Ongoing Project activities include the following:

  • Additional accident studies-- Analysis of several years of data from the Highway Safety Information System (HSIS) accident data files is being conducted to determine if anything definitive can be derived therefrom relative to the impact performance of roadside features in accidents involving light trucks. The researchers are also examining sample data from the National Accident Sampling System (NASS) General Estimates Systems (GES) to determine if this data base can provide additional information on light truck behavior in accidents involving roadside features.

  • Crash testing-- Additional funding has been provided for the project to include a series of tests involving four different light trucks impacting the widely used G4(1S) W-beam guardrail system. In each test the barrier will be impacted at 100 km/h, and at an impact angle of 20 degrees. Vehicles to be tested will include a small utility vehicle, a mid-size utility vehicle, a large pickup truck (the 2000P vehicle), and a large passenger van.

It is expected that the research effort will generate a report that will provide an assessment of the ability of the various roadside hardware to accommodate the light truck class of vehicles. This report will identify those vehicle and hardware combinations where significant incompatibility exists and it research plans to address these needs will be provided. The report is expected to be published in early 1999.


 

FHWA PERSPECTIVE ON UPDATING NEEDS
James H. Hatton, Jr.
Federal Highway Administration

Introduction

Most of the issues to be mentioned here have already been raised by others. The comments in this paper are intended to add perspective and give emphasis to the issues. At the outset it should be noted that I believe that it is important to involve the American Association of State Highway and Transportation Officials (AASHTO) in any update to Report 350. In a recent meeting with people from the national headquarters of the AASHTO, a major concern was expressed about the FHWA’s taking Report 350, a research report created for AASHTO, and making it a standard. This is what the FHWA effectively did. It is important that AASHTO be involved very heavily from the beginning. Members of the AASHTO Task Force for Roadside Safety, which is responsible for the formulation and oversight of AASHTO policies on roadside safety design, need to play a role in the formulation and review of any changes proposed to Report 350.

Tolerances

One of the questions posed has been what should the tolerances be for test installations. These need to be reasonable, but I don't know what the exact numbers should be. For certain, there is a need to accurately and rather precisely report what was tested and what the test conditions were at the test site when testing was conducted. The test installation tolerances should be related to what can realistically be expected real world installations. It is believed that you can get much closer to most nominal dimensions than two inches (50 mm) on most features. An eighth of an inch (3 mm), might be a little tight for most layout dimensions. Designers, suppliers, users, and testers need to be cognizant of the importance of specifying material and installation tolerances. Those who conduct a test should accurately and rather precisely report the actual properties and dimensions of all elements of the test installation, as well as all other aspects of the conducting of a test, that might be expected to influence the test results. It is also important that the specified service tolerances in the materials and installation of a feature be considered in determining if a test installation of a feature adequately represents what might be delivered and constructed under the specifications. It could be that specified service tolerances will need to be tightened to ensure that service installations of a feature will perform as tested or it may be that tests at the limits of the specifications will be needed. For example, the metals currently used in w-beam guardrails are far better than the specification requirements. On the other hand, the thicknesses of the w-beams are very close to the minimum allowed under the specifications. It is obvious that problems could result if tests are conducted on w-beams of typical metal quality and above minimum thickness and service installations are built with w-beams of either minimum allowable metal quality or thickness or both.

Foundations

Soil and foundation conditions, which, in testing of the MELT terminal during the past year, have repeatedly been shown to be influential in test results, certainly must be adequately addressed in the design and testing. These must be reported and evaluated in assessing the acceptability of hardware after testing. Currently, there are no prescribed tests for determining test site foundation conditions. Therefore, a way to test the strength of test installation soils and to make assessments of the relevance of the test conditions to expected service conditions needs to be prescribed.

New FHWA Memorandum and Reporting Requirements

On July 25, 1997, the FHWA issued a new memorandum on the use of Report 350, which replaces a similar, November 12, 1993, memorandum. The title of the memorandum is "Action: Identifying Acceptable Highway Safety Features." Attached to the memorandum is a document, "Background and Guidance on Requesting Federal Highway Administration Acceptance of Highway Safety Features," that describes acceptance procedures and how the FHWA believes they should be applied.

Some surprises might be found in this new memo. One might be the requirement that the wiring in any feature must be similar to the in-service condition, so that the influence of the wiring on the break-away characteristics can be assessed and the potentials of an electrical hazard determined if the feature is knocked down. Another item that might be a surprise is the inclusion of explicit discussion of work zone traffic control devices in the body of the memorandum and its attachment.

Because of the deficiencies in many test reports submitted to the FHWA, the attachment to the memo also includes guidance on what should be recorded and reported when testing features, which is considerably expanded in comparison to that in the 1993 memo cited earlier. All testers and producers should pay particular attention to the emphasis on reporting given in the memorandum’s attachment.

Since Report 350 only gives slight recognition to the use of pendulum testing, it is important to point out that this new memorandum shows FHWA’s continued acceptance of this test method for determining the acceptability of some types of features. The conditions for the use of pendulum tests and the procedures for their interpretation are about as were set forth in the 1993 memorandum.

Report 350 covers reporting requirements and touches on pendulum tests. Nevertheless, it is suggested that the reviewers of Report 350 consider whether the language in Report 350 is adequate to meet the needs indicated in this new FHWA memo. It would be appropriate to add more guidance on pendulum testing if it does not.

Number of Tests Needed

Some people have suggested that Report 350 calls for too many tests. On this matter, consideration should be given to how extensively a feature is likely to be used. If there are truly unique conditions were a special feature is needed and it will only be used a few times, it might be reasonable to consider special analyses and abbreviated testing procedures or the excusing of testing altogether, if on the basis of experience reasonably safe performance can be assured under the most severe service requirements. Most of the complaining about the Report 350 testing requirements heard relates to the cost of bringing a new feature to market, one which they hope will have widespread use. Based on the surprises encountered in test programs, it is hard to agree that too much testing is required. On the other hand, there may be more Report 350 tests, for some features, that should be waved because there is a very low probability they will reveal a problem with a specific feature. Nevertheless, because of the surprises often encountered in test programs, efforts should be made to err on the side of safety when considering excusing tests. What we need to do is to perform enough tests to fully assess the likely field performance of a feature. If several thousand copies of a feature, probably with long service lives, are to be installed a year, the cost of testing the feature should be considered primarily on the basis of the relevance of the testing to long term, systemwide safe performance.

A minor issue somewhat related to the number of tests required under Report 350 is gating versus non-gating terminals and crash cushions. This is considered a spurious distinction and very serious consideration should be given to dropping it in any update of Report 350.

Test Speeds and Test Angles and Test Vehicles

Some have questioned the test speeds in Report 350. There are valid concerns about test speeds relative to the new speed limits and the actual speeds on the highways. There is a need for a serious look at this issue in conjunction with the speed-angle combinations used in testing.

With regard to the speed-angle combination, there is a need to be cautious about the "1.2 percent" that King Mak talked about. If all the fatalities occur in that 1.2 percent of the occurrences, that is where all the work should be focused. That percentile has to be looked at a little more carefully because it represents a low percentage of all encroachments or even all reported crashes.

As Dick Powers implied, test vehicles are of concern in the FHWA. They create problems for the replication of test results if the same vehicle isn't used. At least, different models of test vehicles should be expected to produce identical test results. To get comparable answers, there is a need to more specifically specify test vehicles.

International Testing Procedures and Laboratory Certification

On the issue of international compatibility, it is hoped that some vehicle in the pick-up category (the light truck category) could be found that is common to a number of nations. The mini-van is getting to be popular in Europe. If this is true, it might be a good universal test vehicle. It is certainly very popular here—just look in any parking lot. However, it doesn't have characteristics similar to a lot of the sport and utility vehicles and this could be a problem. The basic criterion for the selection of test vehicles and test conditions should, of course, be that they ensure acceptable feature performance on the highways where the tested features are to be used. This could mean, because of the vehicle mixes in various locations around the world, that a universal set of test vehicles will not be possible. However, it would be desirable to start by arraying all the vehicles used in the world to see if there are enough different vehicles common to all the world’s highways to permit the development of a universally acceptable set of test vehicles.

In working on the issue of international testing compatibility the issue of laboratory certification has arisen. However, this issue also has domestic implications. To ensure confidence in all testing laboratories, a lab certification procedure is needed. If the Report 350 update could make some suggestions in this direction, it would be helpful.

Evaluation and Acceptance Criteria

It is believed that the Report 350 evaluation criteria acceptance levels should be set as close to what can technically be achieved and can be justified. Report 350 does not come as close to following this philosophy for the design and acceptance of safety features as I would have it. There has been some discussion about increasing the complexity of the evaluation values by using a performance measure that combines longitudinal and lateral vehicle motion to obtain a single performance value. This is believed to be partly the result of purists not liking the simplistic, segregated, bi-directional approach used in Report 350 and partly because computers now make a more complex model easy to manage. A more complex approach is acceptable so long as it does not reduce safety or the understanding of what happens in tests. On the other hand, there can be strong opposition to a sometimes mentioned approach that calls for basing acceptance criteria on the assumed use of available occupant restraint systems, unless there is a related reduction in acceptance values. First of all, to recognize the use of restrain systems at this point in time would effectively mean retreating from a standard of performance that many are fairly confident is achievable within our current technical and economic constraints. In addition, the occupant restraint systems are developed to work at much lower speed ranges than have been chosen for the development of highway safety features. Thus, the efficacy of these restraint systems at current highway design speeds is not known.

The recommended acceptance procedures for highway safety features in NCHRP Report 230, developed by Jarvis Michie, presented "limit" occupant impact velocities and ridedown accelerations, which were adjusted by a factor of safety. The proposed factors of safety were 1.33 for both occupant impact velocities (OIV) and ridedown accelerations for all cited features, except the OIV for sign and luminaire supports, for which a factor of safety of 2.67 was proposed, and the lateral OIV for redirected impacts into traffic barriers, for which a factor of safety of 1.5 was proposed. These factors of safety were selected for the safety they would provide and because they were considered achievable. The limit values were set at levels estimated to produce serious but not life threatening injuries (an abbreviated injury scale 3 injury). In truth, the limit values can almost certainly be expected to produce some life threatening injuries at or below the prescribed test impacts and, as already implied, the test conditions don’t cover even all the legal highway speeds. With the adoption of Report 350, the factor of safety concept was jettisoned and the acceptance levels for all features (except sign and luminaire supports) were pushed to the Report 230 limit values, albeit with suggested lower "preferred" values that were almost immediately forgotten. It is strongly recommended that the safety factor concept be reconsidered in the review and update of Report 350. However, this issue might become moot if a serious attempt is made to design roadside features to address the estimated 25 percent of the fatal crashes into fixed objects that involve vehicles sliding side-on into such objects. This is an important consideration in the update of Report 350.

A number of people have found the go-no go nature of the acceptance values in Report 350 troubling. As Dick Powers pointed out, the FHWA has added some precision to the numbers in Report 350 by accepting an occupant impact velocity of 12.2 m/s, where 12 m/s is shown in the report, and a ridedown acceleration of 20.4 gs, rounded to the 20 gs shown in the report. While this might be considered expanding the envelope of acceptance, it does nothing to remove the sharp demarcation between acceptable and unacceptable, probably just the opposite.

Currently there is no way to score the overall crash test performance of a feature to balance good performance results against other results slightly over the specified limit, nor is there a way to factor in expected costs, constructibility, maintainability, or durability of a feature. It may not be possible to come up with a better acceptance procedure than is currently in Report 350. Making it more subjective would open the door to a variety of abuses. Nevertheless, an effort should be made in the update to ensure that the acceptance procedures do not exclude good, highly cost-beneficial safety features.


FHWA PERSPECTIVE ON UPDATING NEEDS
Richard D. Powers
Federal Highway Administration

Since the mid-1980's, the Federal Highway Administration’s (FHWA) Office of Engineering has been the focal point for the review of crash test data and the subsequent determination that a specific roadside appurtenance is or is not acceptable for use on Federal-aid highway projects or on the National Highway System (NHS). This determination is based on a comparison of the crash tests results with the current "standard." Since July 16, 1993, the recognized standard has been based upon the National Cooperative Highway Research Program Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features." This document, the most recent in a series of similar publications, includes specific guidelines pertaining to the vehicles to be used in a testing program, the number and type of tests recommended, evaluation criteria upon which to judge the success of each test, and documentation and reporting requirements. This paper briefly identifies and addresses some of the concerns that have arisen in the four years during which the FHWA has operated under the current standard.

Test Vehicles

The Report 350 test matrix specifies the use of five different vehicle types depending on the test level desired. Test levels 1 through 3 require the use of an 820-kg passenger car and a 2000-kg pickup truck. Test level 4 adds to these two vehicles an 8000-kg single unit truck, and test levels 5 and 6 require the use of a 36000-kg tractor-van trailer and tractor-tanker trailer, respectively. A 700-kg subcompact car is an optional vehicle at each test level, but it has not been used to date.

Due to the increased percentage of vans, minivans, pickup trucks, and sport utility vehicles (SUVs) in the traffic stream in the United States in recent years, the decision was made in writing NCHRP Report 350 to use a 3/4-ton pickup truck as a surrogate vehicle representing the light truck class of vehicles. The 2000-kg (4400-lb.) truck replaced the 4500-lb. passenger sedan that had been the predominate full-sized test vehicle prior to adaptation of Report 350. Subsequent retesting of hardware that had been accepted under earlier test criteria revealed that the pickup truck was less predictable than the full-sized sedan had been. In fact, some safety devices in use for many years along the nation’s highways failed to meet the Report 350 evaluation criteria when tested with the pickup truck.

Two concerns have consequently arisen. The first is whether or not the 2000-kg pickup truck is the appropriate surrogate for the entire light truck class of vehicles. To answer this question, the FHWA is currently testing both larger and smaller SUV’s to determine standard barrier performance with these vehicles. The second concern is directly related to the 2000-kg truck used in the testing process. Report 350 specifies a total weight and some dimensions, but does not require a specific vehicle make or model, nor does it recommend a specific bumper height. Most researchers agree that these variables can directly affect test results since there are significant differences in bumper design and placement and in suspension characteristics between various auto makers. The intent of Report 350 is to test hardware, not vehicles. However, sometimes the distinction between the two is difficult to quantify.

Number of Tests and Impact Locations

Report 350 is, at first glance, explicit in identifying the tests that need to be run on longitudinal barriers and barrier terminals/crash cushions, but some latitude does exist. For example, if a barrier performs adequately with the 2000-kg pickup truck, a test with the 820-kg car may be waived by the FHWA if the geometry of the barrier and the geometry of the car are such that smooth redirection can be reasonably assumed. Similarly, Report 350 permits the omission of some terminal tests if, based on the specific design of the terminal, these tests are demonstrably less severe than others that have been successfully completed.

One topic that often arises is the need to rerun previously passed tests if the design of the hardware has been changed as a test series progresses. Report 350 does not specifically address this concern, but there is agreement among the research community that if any design modification made could change the results of a previous test, that test should be rerun. Unfortunately, there is not always agreement between researchers and reviewers as to what constitutes a significant change from one design to the next. The FHWA’s position is to require a retest if there is reasonable doubt concerning the effect that a design change would have on any earlier successful tests.

The intent of Report 350 is to subject safety hardware to the most critical types of impacts that can reasonably be expected to occur in the field. To meet this purpose, all devices must be tested at the appropriate critical impact point (CIP), which may be defined as the impact location where a device is most likely to fail. Obviously, the CIP is dependent on the specific design of a device and will generally be at different locations for different devices. The recommended procedures for identifying the CIP contained in Report 350 are difficult to follow and, without significant expertise in computer modeling, difficult to corroborate. Report 350 suggests that there may in fact be more than one CIP for a given device and that more than one test may be required in some instances. A more easily applied and readily acceptable method for determining the CIP is needed.

Test Equivalency

Prior to the adoption of Report 350, full scale tests were conducted using earlier test matrices. Each of these differed in some respects from the tests recommended in Report 350. For example, in 1989, AASHTO adopted its "Guide Specifications for Bridge Railings" which included a crash test matrix that is different from the one that is contained in Report 350. The FHWA has considered a Guide Specification performance level two (PL-2) bridge railing, which requires tests with an 820-kg car, a 2450-kg pickup truck and an 8000-kg single unit truck, to be essentially equivalent to a Report 350 test level four (TL-4) railing, which also requires tests with the small car, a pickup truck and the large single-unit vehicle. The main difference is the pickup truck. The Guide Specifications required a 5,400 pound truck that could either be a half-ton or a three-quarter ton vehicle ballasted to 5,400 pounds. The additional ballast changes its weight distribution and its crash performance characteristics significantly from those of the 4400-lb. truck specified in Report 350.

A second important difference in the test matrices is the impact angle. Under Report 350 it is 25 degrees, compared to 20 degrees under the Guide Specifications. That translates into approximately 33.7 percent greater inferred impact severity on longitudinal barriers. So, there is no absolute guarantee that a bridge railing that passed the AASHTO Guide Specification tests would also pass under Report 350. At present, the FHWA requires all new bridge railing designs to be tested under the Report 350 guidelines.

Evaluation Criteria

Report 350 includes a table called "Safety Evaluation Guidelines." These are the criteria by which each test is evaluated and they are generally considered "pass-fail." Most of these criteria are straightforward. For example, a longitudinal barrier must contain and redirect an impacting automobile or pick-up truck remaining upright. However, "moderate" roll, pitch, and yaw angles are acceptable. "Moderate," unfortunately, is not defined. Similarly, deformation of the passenger compartment "that could cause significant injury" is not allowed, but no method of making this determination is suggested, thus creating a very subjective criterion.

In Report 350, driver survivability is based on two calculations: occupant impact velocity and ridedown accelerations. The first criterion covers the forward and lateral speeds with which an unbelted occupant strikes the interior of the vehicle after its initial impact with an exterior object and the second measures the maximum 10-millisecond accelerations of the vehicle from that point until it comes to rest. The maximum Occupant Impact Velocity (OIV) limit is 12 m/sec. The maximum ridedown acceleration is 20 G’s. Since the 12 m/sec value was simply a metric conversion from the previously-used 40 ft/sec (12.19 m/s), the FHWA has accepted 12.2 m/sec as the upper limit. Similarly, since the ridedown acceleration is given as a whole number, a ridedown acceleration value that rounds to 20 G’s is also considered acceptable.

Reporting and Documentation

One of the recurring problems the FHWA has had in reviewing test data is that it often is either incomplete or inaccurate. We must have adequate information to identify specifically what was tested and what the test conditions were. FHWA review process is often significantly delayed while we await additional information or revised drawings. For example, one of the items that Report 350 requires to be included in each test report is the point in the test sequence at which brakes are applied to the test vehicle following impact. This has become an issue because we have the problem of vehicle rebound. If the brakes are locked too soon after impact, the vehicle’s trajectory is going to be affected and it obviously not going to roll very far.

Summary

Although NCHRP Report 350 is more comprehensive than any of its predecessors, its use over the last several years has revealed areas that remain subjective in nature. These include the specific test vehicle to use (e.g., vehicle make, model, and year), the appropriateness of the test matrices (e.g., speed and angle of impact), the location of the "critical impact point" (CIP) for barrier terminals, crash cushions, and transition designs, interpretation of test results (defining the term moderate as applied to roll, pitch, yaw angles, and occupant compartment intrusion), and the perceived need for non-specified tests in some instances.

The FHWA’s goal as a reviewing agency is to be reasonably assured that all safety devices accepted for use on the Nation’s public roads, streets, and highways will perform acceptably under most anticipated crash scenarios. FHWA recognizes that no safety device will perform as intended 100 percent of the time, but by testing all devices to an accepted standard such as Report 350, the probability of success is increased. The likelihood of satisfactory performance is further enhanced if field personnel are made aware of the functional characteristics of each accepted device, thereby enabling them to select, design, install, and maintain each device properly. Finally, the need for in-service performance data on each safety appurtenance in use remains critical, and largely unmet.


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