An inductive sensing unit is used to modify the frequency of an oscillator when a road vehicle passes through the field. An additional circuit stage ambient disturbances e.g. temperature, humidity etc. The output of a constant frequency oscillator (1) drives a parallel tuned circuit (2) consisting of a capacitor and the inductance of the vehicle detecting loop which is tuned to the oscillator frequency. The phase different between the oscillator and the tuned circuit signals is measured by a phase discriminator (3) whose output is amplified (4). The amplified output is fed to a window discriminator (12) and to an operating circuit (5-9). The window discriminator is via an operating circuit dependent compensation inhibitor (15) and an operating influent cing delay circuit (14) to a digital compensating unit (10). The latter connects discrete capacitors (14) across the timed circuit.