Abstract
The travel time is determined along a 5 km long stretch of motorway by means of a number plate survey. This travel time is compared to travel times estimated with two different methods which use the values of traffic flow and mean speed, measured at intervals of 500 m. The simplest method, based on the speed values only, is found to yield the most reliable results in case of both free flow and congestion. In order to obtain a reasonable accuracy the detectorspacing has to be less than 1 km and the sample time has to be 2 minutes.