Road safety has become an important issue and gained much attention for many years. To avoid vehicle collisions on the roads, researchers have proposed various collision warning systems (CWS) using different technologies. Compared with CWS using other technologies, a radio-based CWS is more capable of avoiding collisions in off-sight scenarios (such as the four corners of an intersection). Most existing radio-based CWSs operate using vehicle-to-vehicle communications. However, in such CWSs, high mobility of moving vehicles easily results in message collisions and link breakage, impairing the reliability of these CWSs. An alternative is employing vehicle-to-infrastructure communication to build CWSs. In this paper, we propose a radio-based CWS employing vehicle-to-infrastructure communications. Our simulation results show that the proposed CWS can effectively reduce the number of vehicle collisions without degrading the traffic throughput of a road network.