Vehicle stopping distances and friction measurements.

In Denmark, the allowable traffic speed on certain parts of the Danish motorways was increased from 110 km/h to 130 km/h in 2004. This has resulted in the need to investigate the minimum and maximum geometrical values for road sections or road intersection. To determine the minimum and maxi mum geometric values for road sections, several parameters are important such as: height of eyesight, height of object, height of vehicle, vehicle operator reaction time, vehicle stopping distance, and allowable traffic speed. When one of these parameters is changed, it can have significant influence on traffic safety and in the case as in Denmark, where the traffic speed was increased on some of the freeway network, it is important to investigate the possible impact on the geometrical design and hence on the traffic and traffic safety. The friction coefficient and acceleration and deceleration parameters for vehicles are key factors for vehicle stopping distances and hence vital parameters for traffic safety. Hence one of the first initiatives that were taken, in Denmark, was to increase the minimum allowable friction level on the freeway network. However, realising that the friction level on a pavement can change dramatically within short time and as factors as vehicle braking system and driver behaviour are important factors for the stopping distance of a vehicle, the Danish Road Directorate initiated in 2005 a two year project with the purpose of investigating the influence of different parameters on vehicles' stopping distance. In the Green Book, A policy on geometric design on highways and streets (AASHTO 2001) the determination of braking distance is changed from the more traditional calculation methods using friction coefficient to registration of vehicle operator behaviour and measurements from braking tests. In Denmark, braking distance is calculated based on vehicle speed, friction, and longitudinal gradient. To investigate the method used in Denmark at present and to compare it with results shown in the Green Book, the Danish Road Directorate will in 2006 execute controlled braking tests on several pavements in Denmark. This paper will describe the outline of the test programme, the tests to be performed and the expected impact on the geometrical design guide in Denmark for highways and streets (A). For the covering abstract of the conference see ITRD E212343.