Safety Effects of Geometric Elements in Design Exceptions

The American Association of State Highway and Transportation Officials' (AASHTO) publication titled, "A Policy on Geometric Design of Highways and Streets" (Green Book), provides guidance to the designer by referencing a recommended range of values for critical dimensions for the design of highway facilities. For various reasons, it may be necessary to design a section of highway with substandard values for some elements. Such design exceptions require appropriate assessment and justification of the potential impacts to highway safety and operations. As most design exceptions involve highway geometric dimensions that do not meet the requirements of the design standards, it is essential to understand the safety implications of these geometric dimension shortfalls. This paper presents the results of evaluating the effects of geometric elements in design exceptions on highway safety with the methods provided in the Highway Safety Manual. The safety effects are assessed in terms of expected crash rates of individual substandard elements as well as the combined substandard elements.