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

1. Design Speed


Target areas:  Any highway where a design exception is used for design speed.

Strategy:  Cross-sectional elements to manage speed.

As discussed in Chapter 3, design speed is a design control, and the chosen design speed affects many of the geometric elements of a highway.  Design exceptions for design speed are also rare, for two reasons:  1) the adopted criteria encompass a range of design speeds, which provides a great deal of design flexibility  and 2) design exceptions, when needed, are normally prepared for the specific design elements and not the design control.

In the rare cases when a design exception is used for design speed, one mitigation measure to consider is choosing cross-sectional elements and dimensions that serve to manage operating speeds so they are at or below the design speed.  For example, on a transitional roadway between a rural and urban environment, a more-enclosed urban cross-section with curb and gutter gives drivers a visual cue that they are entering a reduced-speed environment.  It may also feel less comfortable for a driver to maintain high speeds on such a cross section compared to a more-open, rural cross section with full-width lanes and wide shoulders.  Just as design speed is selected by the designer, cross-sectional elements can be chosen that help manage operating speeds.


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Page last modified on April 1, 2019
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