Development of a Design Flexibility Toolkit

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. This study was performed to develop a guideline for design exceptions. The guideline for development and evaluation of design exception projects was developed and presented. The guideline outlines the steps for developing and evaluating design exception projects. The potential impacts of design exception elements to highway safety and operations are listed for the 13 controlling criteria to provide designers with important and easy to use information. The possible counter measures for each of the controlling criteria are listed in a one-page table for easy reference. The proposed safety evaluation process was presented in terms of safety impacts of individual substandard elements as well as the combined impacts of the substandard elements. An Excel based computer program was developed for life-cycle benefit-cost analysis of design exception projects. A decision on design exceptions can thus be made rationally with the recommended guideline and methods.