Vehicle detection systems

A vehicle detection system comprising a transmitter for supplying a continuous wave signal, a receiver and sensing means for coupling said continuous wave signal to said receiver so that the approach of a vehicle produces a disturbance having a leading edge with a changing voltage level in the envelope of the signal received by said receiver, said receiver including first means for deriving from said received signal a first signal representative of the envelope voltage at a first instant during the leading edge portion of said disturbance at which the slope of the leading edge first starts to markedly decrease less a voltage corresponding approximately to the envelope voltage prior to said disturbance produced by the approach of a vehicle, second means for deriving from said received signal a second signal representative of the envelope voltage of the received signal at a second instant at which the slope of the leading edge is substantially at its maximum, third means for deriving from said received signal a third signal representative of the envelope voltage of the received signal at a third instant occurring at a fixed time period subsequent to said second instant and prior to said first instant, means for producing from said third and second signals a fourth signal representative of the difference between the envelope voltages at said third and second instants and means for dividing said fourth signal by said first signal to obtain an output signal indicative of the velocity of the vehicle.