Risk-Based Volume Warrants for Free Right-Turn Lanes on Two-Lane Roadways

Abstract

Establishing warrant conditions for auxiliary free right-turn lanes is not only a policy concern but an engineering judgment that should be based on sound engineering principles. This paper presents a new methodology for establishing volume warrant conditions for free right-turn lanes on two-lane roadways based on a risk probability of a potential rear-end crash involving a decelerating right-turn vehicle and the immediately following advancing through vehicle. With the assumption of shifted negative exponential distribution of arrival interval and application of compound probability theorem, the risk probability can be expressed as a function of the total approach volume and the percent of right turns. By choosing a desired limiting risk probability, warrant conditions for right-turn lanes can be deduced. The rationality of the proposed methodology was evaluated using field survey data, indicating a fair agreement between the modeled probabilities and the field observations. Thus, the proposed methodology established a rational basis for setting volume warrants for free right-turn lanes on two-lane roadways. In application of the proposed methodology, a set of warrant curves was derived, which can be readily used by an engineer in the decision for installation of a free right-turn lane on two-lane roadways.