Speed differential as a measure to evaluate the need for right-turn deceleration lanes at unsignalized intersections.

Limited research is available to help transportation agencies make decisions about installing right-turn deceleration lanes at uncontrolled approaches of unsignalized intersections and driveways. Criteria for right-turn lanes developed by some agencies do not adequately consider the effects of through and right-turn movement volumes and speeds. To evaluate the need for right-turn lanes, the percentage of through vehicles behind right-turning vehicles in the outside (right) lane that perform evasive maneuvers because of the presence of right-turning vehicles has been used as a surrogate. This measure is difficult to use under different combinations of volumes, speeds, and other operating conditions; it also cannot be estimated from traffic simulation models if these models are used to evaluate the need for right-turn deceleration lanes. Safety is the most important factor when deciding to provide a deceleration right lane. The speed differential between through vehicles affected by right-turning vehicles and those not affected by these turns is used to determine the need for right-turn deceleration lanes at unsignalized intersections. The speed differential measure has been shown to be a surrogate to safety and to be related to crash involvement. To determine the total speed differential caused by right-turning vehicles in the outside lane, two variables are needed: the number of through vehicles in the outside lane affected by right-turning vehicles and the average drop in speed of affected vehicles. Total speed differential can be used to evaluate the need for a right-turn deceleration lane.