Cyber-physical systems use sensing, communications, and computing to control the operation of physical devices. Sensing and computing devices have been embedded in automobiles and in the transportation infrastructure. Communications adds a new dimension to the capabilities of these systems. The embedded computers and sensors in both vehicles and the infrastructure will be networked into cyber-physical systems that reduce accidents, improve fuel efficiency, increase the capacity of the transportation infrastructure, and reduce commute times. The authors describe applications that improve the operation of automobiles, control traffic lights, and distribute the load on roadways. The requirements on the communications protocols that implement the applications are determined and a new communications paradigm, neighborcast, is described. Neighborcast communicates between nearby entities, and is particularly well suited to transportation applications.