In phase one, eight Finnish and eight British drivers drove the test route three times: unassisted in summer conditions, unassisted in winter conditions, and with a risk display in winter conditions. The risk display informed drivers about road surface friction with a warning bar that grew longer and changed color with increasing risk. The length of the bar was calculated by comparing actual driving speed to the safe speed for prevailing friction and instantaneous curve radius of the vehicle. The bar was initially green, progressing through yellow to red at the high-risk end. In phase two, 24 British drivers were used to compare the following driver support systems under winter conditions: no support system (i.e., typical winter driving feedback), an advanced driver information system, and the Weather-related Intelligent Speed Adaptation (WISA) system. The advanced driver information system included variable message signs located every 1,312 feet (400 meters) indicating “ICE” if road surface friction was low. The WISA system prevented vehicles from exceeding a safe speed on icy road sections. The safe speed was computed based upon curve radius and surface friction.
In the first phase, average travel speeds in icy winter conditions, 36.5 mi/h (58.7 km/h), were 2.5 mi/h (4 km/h) lower than summer travel speeds, 39.0 mi/h (62.8 km/h). Although the risk display decreased speeds in sharp curves, test drivers suggested a more anticipatory system, which would warn drivers before they encountered a curve or icy area. In this phase, only one driver ran off the road on a sharp icy curve. In phase two, the coverage of low-friction areas was increased and the friction of non-icy sections was lowered to better correspond to normal winter conditions. Five drivers with no support system ran off icy road sections and five drivers ran off the road when using advanced driver information system. No test drivers assisted by the WISA system left the roadway. The average travel speed with no driver support system was 39.4 mi/h (63.3 km/h). With the advanced driver information system, the average speed was 38.8 mi/h (62.4 km/h). Drivers using the WISA system had the highest average speed, 40.2 mi/h (64.6 km/h).
It was concluded that driver adaptation to adverse conditions is not adequate because they cannot accurately assess the degree of road surface friction. The Weather-related Intelligent Speed Adaptation (WISA) system appeared to support drivers in adverse road conditions, increasing safety and travel speeds. More advanced measurement techniques are required to provide drivers with adequate road condition information and to recommend appropriate behavior modification.
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