Factors influencing driving speed

The project consisted of two parts. Part 1 was a study of whether changed speed limits influence driving speeds on road sections adjacent to the change. Reduction of the limit from 90 to 80 km/h resulted in lower speed even on the following section. A similar 'speed generalisation' was observed also on a motorway where the limit was increased from 90 to 100 km/h; driving speed increased on the following section there as well. Part 2 was a roadside study, in which drivers were stopped and interviewed about speed and speed limits, after their speed had been measured over a 3-km section. At the same time, the speedometer was tested. Seventy percent violated the speed limit in a 70 km/h zone. The most important variable explaining speed level was driver age; fewer elderly drivers violated the speed limit. The magnitude of the violations varied with the drivers' estimates of the most comfortable speed. The speedometers indicated on the average 4.4 km/h too high speed at 80 km/h. Only one out of three drivers knew about the error and adjusted their speed accordingly. Therefore, the speedometer error contributes to increased speed variation, and thereby to platoon formation and overtakings.