Relationship between Freeway Flow Parameters and Safety and Its Implications for Hard Shoulder Running

Abstract

Decisions to run traffic on freeway shoulders during peak periods are motivated by the need to relieve congestion. Practicing traffic engineers generally believe that the decreased congestion resulting from running traffic on hard shoulders (i.e., hard shoulder running) is associated with some unspecified degree of improved safety, yet the majority of researchers agree that accident rates increase as the number of lanes increase, even if full shoulders are provided. Despite many years of modern road building, these conflicting views have not been reconciled. This paper first examines the relationship of traffic flow parameters, such as volume, density, and speed, to safety with calibrated performance functions of corridor-specific safety. On the basis of an understanding of this relationship, a possible explanation of the effect on safety of hard shoulder running is formulated. Empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that as flow increases, the crash rate initially remains constant until a certain critical threshold combination of speed and density is reached. Once this threshold is exceeded, the crash rate rises rapidly. This rapid rise in crash rate may be caused by an increase in density without a notable reduction in speed and the resultant small headways that make it difficult or impossible for drivers to compensate for error. This model suggests that during hard shoulder running, crash rates decline because of the lower traffic volume or density per lane and that the safety benefits of a reduced volume or density per lane outweigh the adverse effects of the lack of provision of a full shoulder.