Levels of Safety at Freeway Exits

Abstract

Speed consistency has a strong correlation with safety. The conventional measure Δ V₈₅, which is the subtraction of operating speed on two successive elements, may fall into an ecological fallacy and overestimate the safety level of roadway elements. To attempt to overcome such pitfalls, the 85th percentile individual speed difference 85Δ V and the speed reduction rate 85Δ VR were developed as alternatives to evaluate the level of safety at freeway exits. Speeds of 7,617 individual, traceable vehicles on a freeway mainline, deceleration lane, upper ramp, and lower ramp were collected at 42 exits with radar guns. A three-level criterion is given as a guideline for the evaluation of the level of safety. The safety levels of the 42 exits were evaluated with three speed consistency measures, namely, the conventional Δ V₈₅, the newly proposed 85Δ V, and 85Δ VR. The results showed that Δ V₈₅ had the highest number of good and the least number of poor safety level linkages; 85Δ V produced the least good and the highest number of poor safety level linkages; and 85Δ VR gave an in-between outcome. The ratio of 85Δ V–Δ V₈₅ was 1.42 for the freeway mainline–deceleration lane linkage, 1.68 for the deceleration lane–upper ramp linkage, and 1.98 for the upper ramp–lower ramp linkage. Historical crash data on 31 exits were available for validation. The validation results affirmed the reasonability of the measure 85Δ VR, which considered not only individual speed reduction but also the base operating speed of a vehicle as it approaches an element.