Since the early 1970s traffic management systems at major road works on motorways have come to mean contra-flow: opposing streams of traffic sharing the same carriageway. Prior to that time most road works were carried out using lane closure techniques. However, increasing traffic volumes which exceeded the capacity of such simple systems led to greater road user delay so that systems which provided a greater number of traffic lanes had to be developed for major road work sites. Partial contra-flow, a system in which only a part of the traffic affected by the works is transferred to the other carriageway, provided the answer for some time, but fears for the safety of workmen sharing the same carriageway with high speed traffic led to greater use of full contra-flow. In 1981 extra long crossovers were used to transfer two lanes of traffic simultaneously rather than two seperate single lane crossovers and this system was so successful that is has now come to be accepted as the preferred traffic management layout for major road works on dual three lane motorways. The Transport and Road Research Laboratory assessed the traffic safety performance of the traffic management system used in 1982. Their study included 23 sites on dual three lane motorways of traffic controlled by full contra-flow (with two lane and single lane crossovers) and by partial contra-flow. In all, studies were made of 241 personal injury accidents (PIAs) at roadworks and 166 PIAs suring the same period of previous years at the same sites but without works. The PIA rate per million vehicle km with works averaged over all traffic management systems was 1.5 times the no works rate. Partial contra-flow with a buffer lane was the least safe system, whereas partial contra-flow with a buffer zone was the safest, with full contra-flow lying between. (A)
Abstract