Relationship between speed distribution measures and rural two-lane highway characteristics

Abstract

Understanding the behavior of speed population distributions on rural two-lane highways is needed to address driver error associated with elements of the highway. Previous research has identified a direct relationship between speed variance and accident potential. Therefore, identifying relationships between speed distribution measures of free-flowing passenger vehicles and roadway characteristics could be used to distinguish consistent and inconsistent roadway features. Specifically, the focus of this study was on understanding the relationship between mean speed, speed variance, and roadway characteristics. Horizontal curves from four geographic regions were identified based on selected criteria, and speed data were collected at the midpoint of the preceding tangent and the curve midpoint. The results identified a relationship between mean speed, speed variance, and the roadway characteristics for tangents and horizontal curves. The study also analyzed the speed difference between successive horizontal features and found an average speed reduction based on the curve radius. Mean speed and speed standard deviation were related to the posted speed of the roadway, the length of the tangent, and the radius of the curve. The speed reduction from the tangent to the horizontal curve was a function of the curve radius. The research documents that tangents greater than 250 meters give drivers ample opportunity to reach their desired speeds. Incorporating the 10 km/h speed differential between features suggested by Lamm, a minimum radius following long tangents was recommended. For horizontal curves following tangent lengths greater than 250 meters, the radius should be greater than 150 meters to ensure speed differences between the tangent and horizontal curve are less than IO km/h.