(57) In a multi-channel inductive loop vehicle detection system with environmental tracking, cross-talk between the detector loops is minimised by adjusting the respective scanning periods of the different detectors to substantially eliminate the possibility of two such scanning periods being coincident long enough to produce a detectable interference signal. This is achieved by modifying the scanning period of each detector, from a nominal fixed scanning period, to ensure that no two channels are coincident to produce an interference signal long enough to exceed the environmental tracking rate. By providing an interscanning period timer with a period N calculated to exceed the normal loop oscillator running time and causing the system to wait until that period N has expired before stepping from one channel to the next it can be ensured that the loop oscillator has ceased running before stepping to the next channel occurs.