Speed Levels of Heavy Vehicles on Norwegian Mountain Pass

This paper presents results from a study of how speed varies among vehicle categories under variable winter conditions and how incline affects traffic flow. The study was divided into two parts. The first part was a case study with observations of traffic behavior influenced by weather events. The case study formed the basis for hypotheses about how winter conditions could influence speed. The second part of the study tested some of these hypotheses through information about traffic, climate, friction, and winter maintenance activities over 2 to 4 years. The study found that speeds in all vehicle categories decreased when the coefficient of friction fell below 0.30. If snow was falling as well, the speed decreased further. For trucks, it appeared that snow, in conjunction with a corresponding reduction in visibility and poor driving conditions, reduced speed substantially. On a straight stretch with no incline, there was less difference in speed levels among vehicle categories, and load seemed to have little or no effect at all on speed. Concern arose that the speed level did not decrease more when the coefficient of friction fell below 0.3. This finding indicated that drivers of both passenger cars and trucks drove with elevated risk when the friction was reduced. When it was snowing, it seemed that drivers adapted better to driving conditions.