Geometric delay at non-signalised intersections

Four separate studies of 'geometric delays' at a range of non-signalised intersections have been carried out during the past six years by the Transportation Research Group at Southampton University, under contract to the Transport and Road Research Laboratory (TRRL) and the Department of Transport (DTp). The term 'geometric delay' has been used to describe the extra time required by vehicles as they slow to negotiate an intersection. In the early studies at roundabouts and priority junctions, relationships were developed for geometric delay based on category analysis and linear regression techniques. In the later studies, however, more emphasis was put on the development of a synthetic model to simulate the speed profiles of vehicles as they negotiated the intersections. Relationships were developed for parameters such as minimum speeds and acceleration/deceleration rates, these relationships being based on readily measureable dynamic or geometric variables. The synthetic model has been extended so that it is now applicable to all forms of non-signalised intersections giving estimates of journey time as well as delay. The form and application of the model is described in detail in this report. The main features found to affect geometric delay are: (1) the extra distance which has to be travelled compared with the no-junction alternative; (2) the speed drop that occurs between the approach/exit roads and the internal sections of the intersection. (A)