DRIVER SPEED ADAPTATION TO ROAD CONDITIONS

The aim of this report was to study the speed adaptation of drivers in urban areas with regard to road conditions. Friction between tyre and road surface was measured together with vehicle speed during wintertime on a four lane road and stopping distance was calculated. The following results were obtained: (1) speed is generally decreasing in connection with deteriorating friction (ca 13 km/h (both medium and fast drivers) in the slow lane and 18 km/h (fast drivers) in the overtaking lane). (2) The calculations of stopping distance showed that during periods of slippery conditions more than 15% of the drivers in the overtaking lane would have needed a stopping distance of 200 M. Up to 35 km/h speed reduction was needed for motorists with unstudded tyres in order to maintain the same stopping distance during slippery conditions as during non-slippery. Corresponding figure for studded tyres was 30 km/h. (3) Under certain conditions bridges were found to have lower friction values than the rest of the roadway. (4) Sanding and salting increased the friction values by 0,6 units in the slow lane and 0,25 units in the overtaking lane after 30 min. Although vehicle speed increased, the theoretical stopping distance was reduced by half. /TRRL/