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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

Where are Design Exceptions Required?


Design decision making and approval authority varies based on ownership of the highway in question and its functional role or classification within the nation’s highway system. Broadly, roads can be considered as part of the National Highway System (NHS) or other (non-NHS). Evaluating mitigation techniques and implementing them where appropriate can improve safety and traffic operations on any highway.

The National Highway System

The NHS is a network of approximately 160,000 miles (256,000 km) of highways that are important to the nation’s economy, defense, and mobility. The NHS includes the Interstate system. Other NHS routes are principal arterials serving major travel destinations, highways that provide an important function for national defense, and highways that provide connections to other intermodal transportation facilities, such as airports and seaports. Additional information and State maps of the NHS are available on the Federal Highway Administration (FHWA) website: https://www.fhwa.dot.gov/planning/national_highway_system/.

By federal regulation, FHWA is responsible for establishing standards on the NHS (23 CFR 625). Through the federal rule-making process, FHWA has adopted several American Association of State Highway and Transportation Officials (AASHTO) publications as the minimum design criteria for the NHS (see the following section on "Sources of Design Criteria").

Design exceptions are required on any project on the NHS when design values are used that do not meet minimum criteria. FHWA is responsible for design decisions on NHS projects, specifically including approval of design exceptions. This authority exists regardless of the funding source for the project.

FHWA has developed specific guidance on what constitutes the need for a design exception, and how design exceptions are to be studied, documented, and approved.  This Guide addresses FHWA requirements for design exceptions. For additional information on FHWA’s requirements for design exceptions, see the Federal Aid Policy Guide: https://www.fhwa.dot.gov/legsregs/directives/fapg/0625sup.cfm.

Non-NHS Highways

Non-NHS projects are designed, constructed, operated, and maintained in accordance with State laws, regulations, directives, and safety, design, and construction standards. Therefore, there is no federal requirement for design exceptions on highways and streets that are not part of the NHS, regardless of funding source. However, States are encouraged to analyze situations and document exceptions on non-NHS routes in a similar fashion when design values are used that do not meet their adopted criteria.


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Page last modified on April 1, 2019
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