The study described in this paper identifies and evaluates major contributory causes of reduced capacity at motorway roadworks sites. The results of a survey of throughput at roadworks zones operating at or close to capacity are briefly described. In the absence of incidents, flow breakdown was found in the traffic flow range of 1600 to 2300 pcu/h/lane. The available evidence identified the merge area as the major cause of congestion induced by excess vehicle demand flow. Data describing driver behaviour are presented. Observations confirmed that incidents are the major cause of reduced capacity within roadworks sections. The effect of incident duration and capacity on the time taken for a queue to disperse is demonstrated. A simulation model is applied to determine the capacity in roadworks zones for different layouts and traffic management schemes. The results indicate that the capacity is higher when heavy goods vehicles are restricted to the nearside lane and lane changing is prohibited within the roadworks zone. A regression model for capacity is derived from the simulation results. (A)
Abstract