A radar sensor is an essential part for forward collision warning and avoidance. The radar sensor gives information of relative distance and speed. However, some lane change situations need additional sensors for reliable estimation of relative motion. So, the authors introduce a vision system that can detect interruption by lane changing and give approximate relative distance. Vision data are used to verify radar data, and vision/radar data are used for more accurate estimation of relative motion. The authors perform a vehicle test for the verification of this algorithm.