Close driving : hazard or necessity ?

Recent French experiments on painting chevrons on motorways have sought to prevent close following which is thought to be dangerous. The Transport and Road Research Laboratory (TRRL) is considering similar experiments in England. This paper attempts to rectify the lack of thought as to whether close-following is on balance beneficial or harmful. The French and proposed TRRL experiments concentrate solely on driver attitudes to the markings and on whether driver behaviour is likely to be influenced. The paper reviews the different assumptions for car following which have been used in simulations (particularly applicable to US freeway and tunnel control), and those which would arise from Road Safety publicity. It compares the relative impacts of natural behaviour, the UK Highway Code (see IRRD 802460) safe stopping distances, a time rule such as 2-second spacing and a distance rule such as the French chevrons experiment. It considers the impact that these rules would have on different types of road under different traffic conditions. It raises questions on the balance between provision of capacity and safety. Note is taken of the occurrence of close- following as a contributory factor in police reports of accidents, and considers the extent to which these are soundly based. In particular the issue is raised that reduction of close-following on motorways could reduce the capacity of motorways and reduce the proportion of traffic using them. To some extent this reduction of motorway capacity may represent a reduction in total traffic, which would represent a reduction in accidents at the expense of level of service. More worryingly, if total traffic levels remain constant the reduction of motorway traffic would be reflected in increased traffic on non- motorway roads which are inherently less safe; the conditions under which the technique would lead to a net increase in accidents are identified. The extent to which any such conclusions are affected by imperfect compliance with the markings (or time-spacing rule) is considered.