Relationship between Traffic Density, Speed, and Safety and Its Implications for Setting Variable Speed Limits on Freeways

Abstract

Speed–flow relationships for a typical basic freeway segment are understood well at present and are documented by the successive editions of the Highway Capacity Manual. Recent freeway studies showed that speed on freeways was not affected by low to midrange traffic flow. Increases in flow and density without a reduction in speed have a significant influence on safety. However, current literature lacks constructive discussion of this influence. Empirical examination of the relationship between flow–density, speed, and crash rate on selected freeways in Colorado suggests that as flow–density 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. The rise in crash rate may be caused by flow compression without a notable reduction in speed: resultant headways are so small that drivers find it difficult or impossible to compensate for error and avoid a crash. This paper calibrates performance functions for corridor-specific safety that relate crash rate to hourly volume–density and speed and proposes an algorithm for a variable speed limit intended to slow traffic in real time in advance of a high speed–high density operational regime. Deployment of such an algorithm has the potential to improve safety and reduce travel time variability.