The speeds and flows achieved by traffic on road links, and their relationship with road geometry provide an important basis for Highway Link Design and the economic assessment of road improvements. Changes in driver behaviour, improvements in vehicle performance and other factors make it necessary to review these relationships periodically. The report describes the observations and analysis carried out during 1991 to produce speed/flow relationships for rural single carriageway roads. Speeds and flows were measured for a total of about nine hours on each of 42 sections of road carriageway chosen to be representative of the national road network, both in link geometry and traffic flow conditions. Each road was surveyed at different times of day to ensure coverage of the full range of traffic conditions at each site. In contrast to previous studies, which used the moving observer method, registration number matching at the beginning and end of the road section was employed to provide speed and flow information. The surveys were carried out for light and heavy vehicles separately in good weather conditions during daylight. Geometric parameters were obtained from Ordnance Survey maps and site measurements. The data analysed using step-wise multiple regression to relate the speeds and flows to the corresponding geometric characteristics. In general, the same flow and geometric determinants of speed as the previous study were apparent, although there were some differences in the scale of their impact. Updated relationships connecting speed, flow and link geometry are recommended for both light vehicles and heavy vehicles.
Abstract