Factors affecting speeds on motorways and other limited access dual carriageways in urban areas

In May 1989, Colquhoun Transportation Planning were appointed by the Transport and Road Research Laboratory (TRRL) to investigate the relationship between speeds on urban dual carriageway roads or motorways and their measured geometric and environmental characteristics. The purpose of the study was to integrate as far as possible the urban and rural method of identifying a design speed for motorways and other limited access dual carriageways. The Department of Transport Departmental Standard TD 9/81 (see IRRD 257580) gives design guidance for rural roads. In this report, the development of two linear models for urban motorways and dual carriageways is described. The first model is the recommended model, using the variables junctions, bendiness, road class, and number of lanes. It gives the best explanation of the data and ties in most closely with the rural variables used in TD 9/81. In both models the number of junctions acts as a strong determinant of speed. This is consistent with the use of high to low access criteria and layout constraint for rural roads in TD 9/81. Safety aspects were not considered in this study.