RELATIONSHIPS BETWEEN OPERATIONAL AND SAFETY CONSIDERATIONS IN GEOMETRIC DESIGN IMPROVEMENTS

Traffic operations have an important influence on safety. This paper demonstrates that traffic operational improvement projects can have a positive influence on safety under varied highway conditions. Examples of operational improvement projects on two-lane highways and urban arterials that also reduce accidents are cited. The examples address passing lanes on two-lane highways and use of narrower lanes and center two-way left-turn lanes on urban arterials. These examples primarily concern issues related to the highway cross-section. The safety benefits of these types of improvement projects result partly from an improved level of service and partly from smoother traffic operations with fewer vehicle-vehicle conflicts. Relationships between traffic operations and safety are less well understood for other design elements such as horizontal and vertical alignment. The need for flexibility in geometric design standards to obtain both these traffic operational and safety benefits is illustrated. Further research is needed to establish reliable relationships between traffic congestion (volume-capacity, or V-C ratio) and safety for highway sections and intersections.