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 T 1 ; 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 T 2, and a control system to receive said first and second outputs and to compare said first output at T 1 to said second output at T 2 to identify any differences between said outputs, whereby a target vehicle moving in said stationary target area may be detected.