Abstract
In a field experiment the effect of a traffic data registration system on the speed behaviour of passing drivers was studied. The system is used with air-tubes as detectors. It is important to know how far the road user changes his speed behaviour when he perceives the air-tubes and the measurement-car. It was shown that a combination of the air-tubes and a clearly shown measurement car has a significant effect on speed behaviour, which was a decrease in speed of 4 to 6 km/h. The air-tubes alone produce no significant effect.