Speed data that are used in psychological studies may be obtained from self-reports or from measurements in traffic. Although self-reported data may not give a correct picture of the speed a driver actually would choose on the road they are used for practical reasons. In addition, self-reports are often used in modelling of driver behaviour. When speed data are obtained in traffic usually a single speed measure from each driver is used in analyses. Both of these methods do not take into account the variability in individual drivers' speed in real traffic. Questions may arise as to whether an obtained speed measure is typical of a driver's normal speed. In the present paper an attempt is made to show how stable drivers are in their choice of speed. Repeated speed measurements were obtained to examine drivers' consistency in speed choice. Drivers were observed at two locations along a route or for two days at the same location. Single carriageway roads were used with posted speed limits of 70 km/h and 90 km/h. Results showed correlation coefficients between repeated speed measures from .49 to .81 for free-flowing vehicles. Higher consistency between measures was found where segments of the road were homogenous. Consistency in relation to speed limit varied as a function of roads and direction of travel. From 4 to 41% of the drivers observed legal speeds at both sites and between 43 and 100% travelled over the speed limits at both sites. The results are discussed in terms of using self-reported speed in driver surveys and modelling of driver behaviour. For the covering abstract see ITRD E123193.
Abstract