Douglas W. Harwood
Principal Traffic Engineer
Midwest Research Institute
Kansas City, Missouri

This paper presents an overview of the needs for Simultaneous Vehicle/Infrastructure Design (SVID) from the highway infrastructure perspective. The paper was prepared to stimulate discussion at the SVID Workshop to be held by the Transportation Research Board (TRB) on December 17-18, 1997, in Washington, D.C.

The paper first discusses the goals of highway infrastructure design, the potential benefits of SVID, and the scope of SVID from a highway design perspective. The paper then identifies the vehicle characteristics of interest to highway designers and how highway engineers typically obtain the vehicle data they need. Highway infrastructure issues related to vehicle design are then discussed with an eye to identifying, where possible, current applications of SVID or potential future roles for SVID. Finally, the paper identifies some potential topics for future discussion that might be considered for breakout discussions at the workshop.

In designing highway facilities, engineers must meet a broad set of goals that define the criteria for operation of the highway facility itself and its place in the community. These goals include:

The list includes the both traditional goals of highway engineers—safety, efficiency, and minimum cost—and the considerations of economic, environmental, and social/cultural impacts that are essential to public acceptance of highway facilities.

The objective of SVID is to enable the simultaneous consideration of highway and vehicle issues to assure that vehicle characteristics receive appropriate consideration in highway design and that highway characteristics receive appropriate consideration in vehicle design. A key benefit of SVID would be to improve communication between highway and vehicle engineers. In my experience, there are only limited opportunities for direct interchange of information between highway and vehicle engineers. Highway engineers generally learn about vehicle characteristics from vehicle engineering literature, not from direct contact with vehicle manufacturers or researchers, and I suspect that vehicle designers have only limited contact with highway engineers, as well. This may lead to understanding the past, and perhaps the present, but probably limits our ability to anticipate the future.

To be successful, SVID should provide ways to automate the consideration of vehicle characteristics in highway design and highway characteristics in vehicle design. In highway design, there are many interactions between geometric features (e.g., horizontal and vertical alignment) that cannot be adequately addressed except through application of computer technology, and the same holds for consideration of highway/vehicle interactions. Thus, SVID must lead not just to better communications, but also to computer tools that bring highway and vehicle engineering knowledge together.

The ultimate success of SVID must be judged by whether it results in better highway designs and better vehicle designs than could be achieved by conventional methods.

SVID, by its title, suggests that it deals with the design of highways and vehicles. However, from the highway infrastructure perspective, SVID must be broader than just what is traditionally understood as highway design to be successful. In other words, vehicle characteristics are considered by highway engineers in many contexts other than the design of specific highway facilities and SVID should address those needs as well. The operation of the existing highway system is as important to efficient highway travel as the design or redesign of highways, and the management of operations is heavily dependent on the understanding of vehicle and traffic characteristics. Both design and operations have policy, planning, and research components that need to consider present and future vehicle characteristics.

Another consideration in setting the scope for SVID is that only a few new highways are being built today. It is increasingly rare for a new highway to be built on a new alignment. Instead, must highway design work focuses on reconstruction or rehabilitation of existing highways to meet present and future traffic needs. Thus, SVID clearly needs to address reconstruction and rehabilitation projects, as well as new construction.

Finally, a key element missing from the title of SVID, although not from the scope of the workshop, is the driver. The study of highway/vehicle interactions is incomplete without consideration of the driver and, indeed, it seems that the scope of SVID should be defined to formally address highway/driver/vehicle interactions, rather than just highway/vehicle interactions.

A key element of this paper is a review of the vehicle characteristics of interest in highway design and the current methods of considering those vehicle characteristics. First, it must be recognized that our highways are used by a wide variety of vehicle types including passenger cars, pickup trucks, vans, sport/utility vehicles, trucks (including both single-unit and combination trucks), buses, recreational vehicles, motorcycles, and nonmotorized vehicles, such as bicycles. All of these vehicle types must be considered in highway design and operational analyses although, obviously, the most common of these vehicles (passenger cars) and the largest and heaviest of these vehicles (trucks) receive the greatest attention.

The vehicle characteristics of most interest to highway engineers are those considered in current design and operations policies. These include:

The key highway infrastructure issues that these vehicle characteristics affect are:

The role of vehicle characteristics in each of these issues is discussed below. That discussion should be prefaced by noting that the author is a geometric design and traffic engineer, so pavement and structural issues are undoubtedly addressed less fully than they would be by a specialist in those areas.

HIGHWAY GEOMETRIC DESIGN

Geometric design involves the establishment and refinement of the geometric or dimensional elements of the highway including horizontal alignment, vertical alignment, and cross section. The geometric elements of highways are designed in accordance with established design policies such as the American Association of State Highway and Transportation Officials (AASHTO) Policy on Geometric Design of Highways and Streets,⁽¹⁾ commonly known as the AASHTO Green Book, and comparable policies of individual highway agencies.

Vehicle characteristics are reflected in design policies like the AASHTO Green Book in several ways. The Green Book incorporates a set of design vehicles whose dimensions and characteristics are considered explicitly in design. These design vehicles include a passenger car and several types of trucks, buses, and recreational vehicles. Typically, one of the design vehicles is selected as the basis for design of any particular highway project. The specified characteristics of the AASHTO design vehicles include: vehicle height, width, length, overhang (front and rear), axle and hitch point spacings, and turning/offtracking characteristics. The design vehicles play a particularly important role in the design of intersections, where the consideration of vehicle offtracking and swept path width typically controls the layout of the intersection.

The AASHTO Green Book and other geometric design policies contain many specific design criteria that are based on vehicle characteristics but are not tied to any of the specific design vehicles. Vehicle-related characteristics considered in such design criteria include:

In addition, many established geometric design criteria also contain explicit or implicit specifications of driver characteristics or representative driver behavior.

In a 1990 study for the Federal Highway Administration (FHWA), Harwood, et al. performed a review of current geometric design and traffic control criteria based on vehicle characteristics to determine whether those criteria gave adequate consideration to trucks.⁽²⁾ The review included a total of 13 geometric design criteria and 3 traffic control device criteria based on vehicle characteristics:

Research reviews of this type form the basis for setting geometric design policy and could benefit from greater interchange of information between highway and vehicle designers might bring.

Better geometric design inventory files would be useful in giving both highway and vehicle engineers an overview of current roadway geometrics. Existing geometric data bases such as those in the FHWA Highway Performance Monitoring System (HPMS) and the FHWA Highway Safety Information System (HSIS) provide good data on highway cross section, and some data on highway alignments, but detailed geometric data to address many issues is often lacking. For example, a recent investigation as part of the FHWA Truck Size and Weight study addressed the need for geometric design modifications to existing roadways, ramps, and intersections to accommodate larger trucks that might be permitted at some future date.⁽³⁾ Except for data on mainline roadway horizontal curves and grades form the HPMS data base, existing geometric design inventory data to perform this evaluation was completely lacking and had to be obtained laboriously from aerial photographs, as-built plans, and other sources.

An example of SVID activity related to highway geometric design that is currently underway is the development of the FHWA Interactive Highway Safety Design Model (IHSDM).⁽⁴⁾ The IHSDM will be a computer tool that can be used interactively by highway designers in a Computer-Aided Design (CAD) environment to assess safety as part of the design of a highway facility. This assessment of safety will be accomplished to some extent through consideration of historical or predicted traffic accident experience, but to a larger extent the safety assessment be based on examination of site-specific roadway/driver/vehicle interactions that was never possible before in traditional highway design practice.

The IHSDM will include:

All of these modules will work interactively with the CAD system being used by a highway designer, so that the design can be evaluated as it is developed, and all of the modules will need to be capable of passing information to one another.

The development of IHSDM by FHWA is a multi-year, multi-million-dollar effort. The first priority is being given to design of rural two-lane highways because they appear to present the greatest opportunities to improve safety. However, other highway types will be added in the future. While not yet complete, IHSDM is an SVID success story in the making.

Another area of highway engineering that is dependent of information concerning vehicle characteristics is the evaluation of the traffic operational efficiency of the roadways and intersections and the design of appropriate traffic control systems. Traffic operations and traffic control evaluation must obviously be based on data about the current or anticipated volumes and types of vehicles on particular highway facilities. Key vehicle characteristics that play a part in traffic operations are vehicle length and acceleration/deceleration capabilities. These characteristics may be considered explicitly through specified criteria in the Manual on Uniform Traffic Control Devices for Streets and Highways⁽⁵⁾ or implicitly through sources such as the passenger car equivalents (PCEs) of specific vehicle types in the Highway Capacity Manual.⁽⁶⁾ Acceleration and deceleration capabilities are particularly hard to account for properly because of the mix of old and new vehicles in the current vehicle fleet and the highway engineer's lack of familiarity with what the future may hold in vehicle and engine design.

Safe design of the highway roadside is dependent on vehicle characteristics, but highway engineers face challenges concerning what vehicles to design for and how the characteristics of those vehicle are changing over time. There are challenges in roadside design in that the optimal roadside hardware for one vehicle type may not be suitable for other vehicle types. Key tools in roadside design are:

There appears to be a great opportunity for SVID applications in the roadside design area to assure that roadside hardware designs and vehicle designs are compatible so as to minimize the collision damage to vehicles and the risk of injury to vehicle occupants.

Pavement design is based on consideration of the volumes and weights of vehicles, particularly trucks, that will be traversing the pavement. The volumes and weights are expressed in terms of the frequency of Equivalent Single-Axle Loads (ESALs), each representing the passage of a single 18,000-lb axle. ESALs are also considered in forecasting future pavement conditions in pavement management systems. The frictional properties of the pavement wearing surface must also be considered in pavement design and pavement rehabilitation.

The design of bridges and other highway structures are also based on the numbers and weights of vehicles using the structure. Structural design must obviously use very conservative assumptions and factors of safety to avoid structural failure. However, vehicle characteristics other than vehicle weights do not have a large role in structural design.

The workshop will be conducting further discussion of the SVID concept. The highway-related issues that appear to merit further discussion are;