Braking distance, friction and behaviour

af Poul Greibe


The Danish Road Directorate has conducted a study designed to shed light on braking behaviour among non professional drivers and their braking distance at different speeds. The study will be included in an assessment of the current values for braking distances recommended in the Danish Road Standards and Guidelines.

The study was planned and carried out by Trafitec, in association with the Danish Road Directorate.

Background and object

Braking distance is applied as a significant basic parameter in e.g. calculations of stopping sight distance.

A vehicle’s braking distance depends on a number of factors pertaining to the vehicle, the road and the driver’s behaviour. The most important factors are:

·   speed of the vehicle

·   coefficient of friction between tyres and roadway

·   driver’s braking behaviour/technique

·   vehicle’s braking system and condition

·   tyre pressure, tyre tread depth and road-holding capability

·   road’s vertical grade

All these factors affect braking distance to a greater or lesser extent depending on the actual conditions present when decelerating. A general method for determining braking distance at different speeds, which is representative for the composition of cars, drivers and friction, requires knowledge of the significance of the individual factors for overall braking distance. The present Danish Road Standards and Guidelines contain a method for calculating braking distance at different speeds which is essentially based on early American findings.

The purpose of this study is to assess the braking behaviour of non professional drivers, including braking distances under different physical conditions. The findings will be included in an assessment of the present methods for calculating braking distance.

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