Field trials in three European countries, the Netherlands, Spain and Sweden were carried out in order to investigate the eects of an in-car speed limiter. The trials were carried out on urban and rural roads including motorways. A so-called unobtrusive instrumented car was used, where all the measuring equipment was hidden. All the speed limit categories in the respective countries, ranging from 30 to 120 km/h were included. The results showed that the eects of the limiter were greatest in free driving conditions outside platoons. However, the limiter also had eects in congested trac. Momentary high speeds were suppressed eectively, which resulted in less variation of speeds. Approach speeds at roundabouts, intersections and curves became smoother, car-following behaviour became safer in the speed range of 30±50 km/h. On the other hand, in the speed range of 70±90 km/h a slightly higher number of short time-gaps suggested less safe car-following behaviour. Other negative behavioural eects were slightly increased travel time and the increased frustration and stress for the drivers caused by the limiter. The majority of the subjects accepted the speed limiter as a driver-operated system. Half of the drivers would accept the limiter voluntarily in their cars.