The objectives of this study were to: a) summarize past design exceptions to document their frequency and reason for their use and b) determine if any adverse safety implications can be related to adopting design policies and practices related to design exceptions. The design exception file maintained by the Kentucky Transportation Cabinet was obtained with data obtained relating to the types of project, exceptions requested, and the reasons for the various exceptions. A site visit was made to a large number of the locations where the project was built with the requested design exception. Available crash data were obtained at these locations. There was an average of 39 exceptions per year from 1993 through 1998. The majority of the projects involved a bridge replacement with the next most frequent being roadway widening reconstruction projects and construction of turning lanes. The most common design exception was for a design speed lower than the posted speed limit followed by a reduction in either the sight distance, curve radius, or shoulder width. The most common reason for the request referred to the existing conditions on the road followed by the right-of-way issue and project cost. The crash analysis showed that, with a very few exceptions, use of the design exception process did not result in either construction of projects with high crash rates when compared to average statewide rates or an increase in crashes after construction compared to before construction. The analysis showed that the design exception projects resulted in an improvement over the prior condition although some aspect of the design may not be typical. The reasons for the design exceptions have been well documented, and there is no evidence that construction of projects with a design exception had an adverse effect on highway safety.
Abstract