Safety consequences from design flexibility in rural to urban transitions

Roadway design could be viewed as a balancing act of the various elements and constraints that are present and is very specific to the situation in which they have to be considered. This need is even greater for roadways that pass through rural communities because of the need to reduce speed upon entering the built-up sections and to address the needs of through and local users. Therefore, such projects may require the use of flexibility in design and variation from the normally used values or traditional solutions while utilizing the inherent existing flexibility within the Green Book. This paper presents a recently completed research through the identification and selection of cases studies throughout the nation which were projects where flexibility was applied for roads that transition from a rural to a built-up area or pass through a built-up area on a predominantly rural section of roadway. The findings indicate that the relatively minor exercises of design flexibility, as practiced in 22 case studies, produced very few discernable effects on safety. Multiple years of crash data for each project were collected and each site-specific crash was examined (using before and after analysis and an expert panel) to assess the potential for safety consequences related to the design flexibility exercised. Based on these data and case study analyses, it is clear that staying within the range of Green Book design values (or using slightly less desirable values through design exceptions) provided an acceptable level of safety.