A detecting device for detecting moving target vehicles from a moving host vehicle, comprising a first detector means mounted to the host vehicle and aligned to a stationary target area for generating at least a first output representative of the stationary target area at least at a time T1, a second detector means mounted to the host vehicle and aligned to the stationary target area for generating at least a second output representative of the stationary target area at least at a time T2, and a control system to receive the first and second outputs and to compare the first output at T1 to the second output at T2 to identify any differences between the outputs, whereby a target vehicle moving in the stationary target area may be detected.