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