1) Field of the Invention
The invention relates co a method for measuring the speed of a vehicle by transmitting a wave beam directed at the vehicle, receiving a part of the wave beam reflected by the vehicle, comparing the reflected beam part with the transmitted wave beam and deriving from this comparison the speed of the vehicle. Such a method is generally known and is frequently used by police in the form of radar speed measurements.
2) Description of the Prior Art
In the known method a radar beam is transmitted at an acute angle to a road axis from a fixed, generally concealed position along the road. When a vehicle passes through the beam a part of the beam is reflected. This reflected part is received at the measuring point and the frequency of the reflected beam is compared with the (exact) frequency of the transmitted beam. Because the beam is reflected by a moving vehicle it will undergo, as a result of the Doppler effect, a frequency change which is a measure for both the speed of movement and the direction of movement of the vehicle. When it is found from the comparison of the transmitted and received frequency that the speed of the vehicle exceeds the maximum allowed speed at that location a warning signal is generated. This signal is generally used to activate a camera whereby a recording is made of the vehicle committing the speeding offence. This recording then also shows the relevant data such as date and time of the recording and the measured speed.
Problems occur in the known method when it is applied on roads with a number of traffic lanes, such as motorways. Here several vehicles may pass through the measuring beam simultaneously. If one of these vehicles commits a speeding offence it is often not possible to determine indisputably which vehicle this was. When the speed measurement is combined with making a recording, complicated and time-consuming processing of the recording is required in order to determine the offender therefrom on the basis of secondary indications.