Introduction

Introduction to Mitigation Strategies for Design Exceptions

Mitigation Strategies for Design Exceptions was developed to provide designers with practical information on design exceptions and strategies that can be implemented to mitigate their potential adverse impacts to highway safety and traffic operations.

Design criteria, established through years of practice and research, form the basis by which highway designers strive to balance cost, safety, mobility, social and environmental impacts, and the needs of a wide variety of roadway users.  For many situations, there is sufficient flexibility within the design criteria to achieve a balanced design and still meet minimum values.  On occasion, designers encounter situations with especially difficult site constraints and an appropriate solution may suggest the use of design values or dimensions outside the normal range of practice.  In such cases, a design exception may be considered.

Designers should recognize, however, that design exceptions have the potential to negatively affect highway safety and traffic operations.  For this reason, consideration of a design exception should be deliberative and thorough and a clear understanding of the potential negative impacts should be developed.

If the decision is made to go forward with a design exception, it is especially important that measures to reduce or eliminate the potential impacts be evaluated and, where appropriate, implemented.  This guide presents and illustrates a variety of mitigation strategies, including real-world case studies from several States.

Mitigation Strategies for Design Exceptions is organized as follows:

Chapter 1 provides basic information on design exceptions.  Also discussed are the concepts of nominal and substantive safety, which are fundamental to the topic of design exceptions, their mitigation, and decision making.

Chapter 2 discusses the steps of an effective design exception process.  A standard procedure is not prescribed; rather the activities that are fundamental to an effective design exception process are discussed.  Guidance on design exception documentation is included.

Chapter 3 clarifies the 13 controlling criteria, including when design exceptions are required, how safety and traffic operations are affected by the 13 controlling criteria, and what the potential adverse impacts are if design criteria are not met.  Information on substantive safety is provided where available to help designers quantitatively evaluate the expected safety performance of design exceptions under consideration.

Chapter 4 presents and illustrates potential mitigation strategies for the 13 controlling criteria.  The strategies are summarized, by criterion, in Table 22 beginning on page 67.

Chapters 5 through 8 are case studies that illustrate how several States have effectively approached projects with difficult site constraints and design exceptions, including implementation of mitigation strategies.

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