Over the past two decades, automobile crash-related fatality and injury rates have declined over 34 and 40 percent respectively, due in part to improvements in automobile safety. To further improve traffic safety and provide other transportation benefits, the Department of Transportation (DOT) is promoting the development of vehicle-to-infrastructure (V2I) technologies. Among other things, V2I technologies would allow roadside devices and vehicles to communicate and alert drivers of potential safety issues, such as if they are about to run a red light. GAO was asked to review V2I deployment. This book addresses the status of V2I technologies; challenges that could affect the deployment of V2I technologies, and DOT efforts to address these challenges; and what is known about the potential benefits and costs of V2I technologies.