The most common type of intersection improvement considered by many state DOTs is signalization. The Washington State Department of Transportation (WSDOT) uses a system called the Intersection Priority Array, which was originally developed by Ching. This system provides a tool for objectively considering numerous intersections. Although the system is useful, it only addresses the need for and relative priority of a signal. It does not address other actions that may improve the safety and efficiency of the intersection. The goal of this research project was to develop a system for analyzing the need for left- and/or right-turn lane improvements to an intersection and prioritizing the severity of that need.
Development of the system was based on two questions it would have to answer about the intersection: 1. Is a left- or right-turn lane recommended for a particular intersection? This question is answered on the basis of traffic conditions and accident history. Threshold values for volume and accident history are determined from published engineering studies. 2. How severe is the need for a turn lane compared to other intersections being considered? To answer this question, the system assigns dollar values to delay conditions and accident history specific to the intersection. Dollar values are assigned to accidents over the worst 12-month period in a 3-year accident history. The system then calculates the reduction in delay that would result from installing the left- or right-turn lanes by using regression equations from published engineering studies or standards such as the Highway Capacity Manual.
The scaled sum of the accident and delay costs is the severity score for the specific intersection improvement.
The benefits of this system are that it is an objective method of ranking intersections against others and it is easy to use. It requires data that are easily obtainable from resources available at most traffic offices.