U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
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.