Determination of braking distance and driver behaviour based on braking trials

Poul Greibe, Paper from TRB Annual Meeting, Januar 2008

Stopping sight distance is an important design parameter which influences e.g. geometric design, road safety, construction cost. The stopping distance consists of the reaction distance and the braking distance. The recommended braking distances in the Danish Road Standards and Guidelines are in the main based on earlier American findings. In order to be able to assess the validity of the recommended braking distances in relation to contemporary vehicles and motorists in Denmark, the Danish Road Directorate has conducted a study designed to shed light on braking behaviour and braking distances among ordinary (non professional) motorists, at different speeds.

This was done through a measurement programme in which 22 test drivers performed braking manoeuvres at different speeds (80, 110 and 130 km/h). Both professional and non professional test drivers were used in the trials. Two different recent cars with ABS brakes were used as test cars. The braking manoeuvres were carried out on dry and wet road on three test tracks with different friction. The measurement program consisted of a total of 172 emergency stops and 23 comfort braking manoeuvres.

For professional test drivers, a good correlation was found between braking distance on wet road, friction and initial speed. The braking distances for non professional test drivers were in average 20-30% longer compared to the professionals, but also showed large variations.

Based on the findings plus knowledge concerning the significance of other parameters for braking distance (condition of tyre, vehicle, etc.), we have sought to establish a new set of recommended braking distances.

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