Relationship between Freeway Flow Parameters and Safety and Its Implication for Adding Lanes

The decision to add lanes to a freeway is motivated by the need to relieve congestion. Practicing engineers and planners generally believe that the decreased congestion that results from adding lanes is associated with some degree of improved safety, yet the majority opinion of researchers is that accident rates increase as the number of lanes increases. In more than 70 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, with safety by calibrating corridor-specific safety performance functions. On the basis of an understanding of this relationship, a possible explanation of the effect that adding lanes has on safety is formulated. An empirical examination of the relationship of flow, density, and speed to the crash rate on selected freeways in Colorado suggests that, as the 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 rapidly rises. The rise in the crash rate may be because an increase in density without a notable reduction in speed produces headways so small that it becomes difficult or impossible to compensate for driver error. This model suggests that, after the construction of additional lanes, crash rates initially decline because of the lower traffic volume and density per lane. However, as development and rerouting occur, freeways with more lanes are expected to have higher crash rates that are attributable to the increased opportunities for lane change–related conflicts.