Analysis of Operation and Safety Characteristicsof Lefi-Turn Lanes

Left-turn lanes at nonsignaiized intersections on two-lane interurban roads reduce the number of traffic interferences and increase the level of service for the through traffic. These improved conditions result from the opportunity turning vehicles have to decelerate and wait in a separate lane for a suitable crossing gap. Decreasing the number of interferences to through traffic reduces the number of stops and delays, improves energy consumption and environmental conditions, and increases the level of safety at intersections. From a traffic or safety viewpoint, it would be difficult to imagine circumstances in which an intersection without a left-turn lane would be preferable to an intersection with such a lane. There are significant differences, therefore, between left-turn-lane warrants and warrants for other installations, such as intersection signalization. In the case of intersection signalization, there are traffic and safety circumstances (e.g., low traffic volumes) in which signalization is not recommended. In the absence of a trade-off between cent radicting traffic and safety factors, the problem of warrants for turning lanes is based on priorities and resource allocation. Under no budget restraints, the designer will recommend construction of a turning lane at each intersection. With budget restrictions, however, the designer will recommend priorities based on warrants after taking into consideration construction and maintenance costs and such factors as travel time, energy, safety, and environmental effects. In engineering practice, decisions regarding left-turn lanes are often made without rigorous economic evaluation, being based on the absolute value of one or more traffic characteristics at the intersection (e.g., traffic volumes, accidents, or low level of service). In addition, it is common practice to develop different criteria for new and existing intersections, with less severe criteria used for new intersections. The final decisions are likely to vary from country to country in accordance with economic ability and national priorities. But the input to the decision-making process (delay times, number of stops, energy consumption, safety and environmental effects) is similar for all decision makers, regardless of country. This article analyzes the different input variables that the decision maker uses in preparing the warrants for a leftturn-lane installation. These quantitative variables are usually evaluated according to the decision maker’s society, preferences, and economic priorities. The first part of the article presents an analytical analysis of the turning process and its influence on through vehicles. Next, a simulation model that quantifies the aggregative operational effects of turning vehicles is described. This model was developed because of the complexity of the turning process and the inability of the analytical method to provide solutions. The quantitative results obtained from both the analytical and simulation models are analyzed, with special emphasis placed on the number of stops and decelerations.