An Empirical Investigation of the Impacts of Sun-Related Glare on Traffic Flow

Visibility is one of the basic needs for safe driving. Any type of reduction in visibility can lead drivers to change driving behavior resulting in changes in flow or speed, disorder on the network as a result of braking or driver error, or in the worst case to an incident or a crash. Glare can be caused by sun, headlights of other vehicles or other light sources can cause substantial reductions in vision performance. For freeway operations, glare is primarily associated with headlights from vehicles traveling in the opposite direction at night, is often addressed with median barriers or glare screens. However, the effect of sun-related glare is similar to the impact of headlight glare—it can lead to degradation of drivers’ vision and affect driving performance. The occurrence of glare will vary according to time (only on certain days and times when the sun is low in the sky) and location (only on particular combinations of roadway horizontal and vertical geometry). Rather than investigating any direct relationship that may exist between reported crashes and glare, the objective of this paper is to examine the potential impact between measured traffic flow characteristics and the potential for vehicles to be impacted by sun glare. A method based on multiple regression was used to test the effect of the glare issue caused by sunlight on freeway speed/flow and flow/occupancy relationship.