Design of Freeway Speed Change Lanes: Safety-Explicit Approach

Highways, designed mainly for mobility purposes, produce some unwanted side effects, the most serious of which is road collisions. In addressing this concern, the design guides strive to provide designers with a set of guidelines that would help achieve a design with optimum safety performance. Recently, focus has been oriented toward developing safety-explicit relationships for the evaluation of the impact of highway design decisions. Among the major areas for driver decision, and subsequently areas for potential errors and collisions, are the freeway entrances and exits, whether located in a weaving or a non-weaving section. Adequate lengths between ramp terminals, with enough length for speed change lanes, would give drivers enough time to find acceptable gaps, accelerate or decelerate, and merge or diverge properly. This paper presents modeling efforts related to developing safety-explicit relationships for use in designing freeway speed change lanes. The developed safety-explicit models are then presented as design aids for the length of acceleration and deceleration lanes based on expected collision frequency.