Development of Relationships between Safety and Congestion for Urban Freeways

The development of relationships between safety and congestion is documented; traffic volume, speed, and crash data by 15-min periods for urban freeways in the areas of Seattle, Washington, and Minneapolis–Saint Paul, Minnesota, were used. The safety–congestion relationships were developed to provide a basis for estimating the safety effects of design treatments intended to reduce nonrecurrent congestion and increase travel time reliability. The relationships were developed with traffic volume and speed data from detectors in individual lanes for 564 urban freeway segments during study periods of up to 3 years. Traffic crash records were obtained for the same time periods as the volume and speed data. The U-shaped relationship between crash rate per million vehicle miles of travel and traffic density (the level-of-service measure for freeways) was found, with the highest crash rates at low and high traffic densities and the lowest crash rates in the middle range of traffic densities. The high crash rates at lower traffic densities represent predominantly single-vehicle crashes, whereas the high crash rates at higher traffic densities represent predominantly multiple-vehicle crashes. Quantitative safety–congestion relationships were developed for the range of traffic densities from 20 to 78 passenger cars per hour per lane, corresponding to the level-of-service range from C to F.