Effects of curvature on vehicle/driver behaviour

This study is concerned with the behaviour of the vehicle/driver combination on open road-curves. The associations between behavioural parameters and geometry, traffic flow and environmental factors are examined in order to evaluate current British driving practice. All the curves considered had radii less than 500 metres and overlapped limits used in current British design standards. Information on the performance of vehicle/driver combinations found was collected at a total of 56 directional single carriageway and 22 directional dual carriageway public road sites located throughout Great Britain. Cars and goods vehicles were treated separately and a total of about 10,500 vehicle movements were studied. In addition a test vehicle/driver combination was used to collect information at a large number of sites to relate between-site variation to other factors. A series of bivariate and multivariate linear and curvilinear models were fitted to the data and relationships between behavioural parameters and curve geometry, traffic flow and environmental parameters were determined. Highly significant associations were obtained between speeds on the curve/lateral acceleration and curvature. Speeds on the approach link weLe also found to be a significant determinant of vehicle/driver behaviour around open road curves. Other geometric parameters such as verge width, road width, sight distance and curve length were found to have only marginal effects. No significant difference was observed between left and right-hand curves nor between uphill and downhill conditions. Driver behaviour appeared to be in accord with the current design practice for alignments based on design speeds of 85.0 k.p.h. or more. For alignments with lower design speeds drivers were found to adopt speeds in excess of the design speed.