Design speed is a selected speed used to determine the various geometric design features of the roadway.

The selected design speed should be logical with respect to the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway.

Design speed is different from other controlling criteria in that it is a design control, rather than a specific design element. The design speed establishes the range of design values for many of the other geometric elements of the highway.

In selection of design speed, every effort should be made to attain a desired combination of safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social or political impacts.

A roadway of higher functional classification may justify a higher design speed than a lower classified facility in similar terrain. A low design speed should not be selected where topography and other factors are such that drivers are likely to travel at high speeds.

Some factors to consider when choosing a design speed include: