Vehicle-to-vehicle (V2V) communications play a critical role in enabling numerous important cooperative safety applications. V2V safety communications rely on broadcast of self-state information (e.g., position, speed, and heading) by each vehicle, which allows a vehicle to track its neighboring vehicles in real time. One of the most pressing challenges in this research is to maintain acceptable tracking accuracy of neighboring vehicles while avoiding congestion in the shared communication channel. In this article we describe the evaluation of a transmission control protocol that adapts the message rate and transmission power for V2V safety communications. This protocol has been implemented on V2V test vehicles with wireless radios and integrated with existing active safety applications. The testing and evaluation results show that proposed communication design works well in practice, its performance matches the observations from previous simulations and shows great promise for a large-scale deployment of V2V cooperative safety systems.