Statistical modelling of wintertime road surface friction

Prevailing road surface conditions, the grip between tyres and the road surface, strongly correlate with traffic accident rate. Surface friction is reduced especially during snowfall or icing. Friction observations derived from Vaisala's optical DSC111 sensors, operated during two winter seasons (2007/2008, 2008/2009) at several Finnish roadside stations have been used. The devices measure the depth of water, snow and ice on the road surface and also produce an estimate of prevailing friction. The observations have been used to develop statistical equations to model road surface friction. The model has been evaluated against an independent dataset from winter 2009/2010. The goal thereafter was to integrate the scheme into the road weather service of the Finnish Meteorological Institute by combining background information from a road weather model with the derived statistical equations.