Substantial simulation work has been done showing that traffic shock-waves can be reduced or eliminated through "cooperative adaptive cruise control" and "communicating cruise control" where following vehicles are warned that leading vehicles are slowing in time to react. To be effective, however, such devices must be installed and operate in the majority of vehicles in the queue. Even if mandated today, it would take many years for this to occur, due to the development cycle of car manufacturers and the fact that cars typically on the road for about ten years. We propose such a system in which the necessary hardware comprises an addition to after-market PNDs (GPS Personal Navigation Devices) and pays for itself by providing more current information about traffic conditions than would otherwise be available. Users will buy it because of the improvement in quality of traffic reporting, and traffic reporting services will subsidize it in order to improve the timeliness of their data. We propose to build prototypes of this device and to conduct a series of tests in live traffic to demonstrate its efficacy in Phae I. This will provide a basis for more extensive trials in Phase II, and engagement with PND manufacturers.