U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
The consideration of safety is arguably the central issue involved in a decision to accept or approve a design exception. Understanding the relationship of safety to the criteria, the design process, and a desired or expected outcome of the design is important. The concepts of nominal and substantive safety are fundamental to the topic of design exceptions and their mitigation.
The concept of nominal safety is a consideration of whether a roadway, design alternative, or design element meets minimum design criteria. According to this concept, a highway or proposed design is considered to have nominal safety if its design features (such as lane width, shoulder width, alignment, sight distance, etc.) meet the minimum values or ranges. The measure of nominal safety is simply a comparison of design element dimensions to the adopted design criteria.
As an example, the criterion for Interstate lane width is 12 feet. A design alternative that proposes 12-foot lane widths suggests a nominally safe design, whereas an alternative that proposes 11-foot lane widths would not.
Nominal safety is an "either–or"–a design feature or roadway either meets minimum criteria or it does not. Highways built to satisfy at least the minimum design criteria may be referred to as ‘nominally safe.’ By definition, a design exception is the acceptance of a condition that does not meet nominal safety.
In actuality, the safety effects of incremental differences in a given design dimension can be expected to produce an incremental, not absolute, change in safety. The nominal safety concept is limited in that it does not examine or express the actual or expected safety performance of a highway. This second dimension of safety is critical to making good decisions regarding design exceptions.