TRB’s National Cooperative Highway Research Program (NCHRP) Research Results Digest 306: Identification of Liability-Related Impediments to Sharing §409 Safety Data among Transportation Agencies and a Synthesis of Best Practices examines liability risks associated with sharing safety data among transportation agencies pursuant to Section 409 of Title 23, U.S.C. The report also explores best practices; reviews the Pierce County, Washington v. Guillen decision and its potential impact on managing state liability risk; and describes potential strategies for overcoming the impediments to data sharing, specifically those related to liability. The appendixes to NCHRP Research Results Digest 306 was published as NCHRP Web-Only Document 89.