Many of the motorways constructed during the 1960s and 1970s are now approaching the end of their design life and it is estimated that 100 miles of motorway will require renewing annually. The Department of Transport is also engaged in a 4 billion pound programme to widen 1000 km of motorway and trunk roads. These roadworks are being undertaken during a period of continuing rapid growth in the volume of traffic using the motorway network. In order to reduce inconvenience to drivers and minimise delays the Department of Transport have developed improved traffic management systems to make optimum use of available road space. Consequently, within roadworks sections, traffic often moves along narrow lanes and without a hard shoulder. In these circumstances incidents often cause a substantial reduction in throughput which may persist for prolonged periods after the cause of the incident has been removed. The paper describes observations confirming the extent and duration of congestion caused by incidents within motorway roadworks. The dependence of the periods of queuing, which typically may dominate operation where total demand flow exceeds 3000 vph, on the duration of the incident and the time taken for throughput to recover to the level previously achieved is demonstrated. A simulation program which models lane changing on multi-lane highways has been applied to evaluate the effect of incidents within roadworks sections. It is shown that forced merging, close to the beginning of the taper, serves to continue flow breakdown and congestion within the roadworks section after the cause of the incident has been removed. The benefits of controlling density, for example by encouraging early merging on the approach to roadworks, are demonstrated. (A)
Samenvatting