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 envelope rising to at least a first peak level, said receiver including:

• first means for deriving from said received signal a first signal representative of the maximum slope of said leading edge of said disturbance at a first instant preceding the occurrence of said first peak,
• peak detecting means for deriving from the received signal a peak signal representative of the first peak of the envelope voltage during the leading edge portion of said disturbance less the envelope voltage prior to said disturbance, and
• means for producing from said first and peak signals a signal representative of the ratio between said first and peak signals which is proportional to the velocity of the vehicle.