In order to achieve a safe automotive society, it is necessary to develop an on-board intelligent system by the Inter-Vehicle Communications (IVC). We all want to avoid the vehicular collisions under almost all situations such as intersections, highways, and blind corners by using IVC technology through wireless ad hoc networks. The objectives of this paper are to develop the Vehicular Collision Avoidance Support System (VCASS), and to show its performance by various experiments with two real vehicles.We have install personal computers, telecommunication equipments, and GPS in two vehicles. In this system, a vehicle exchanges the location information and the velocity vectors by the UDP broadcast with each other, and calculates the relative positiodspeed. Vehicles make the warning for drivers with the possibility of the collision. We verify the validity and the effectiveness of the VCASS hy the experiments.