Design speed is defined as “a selected speed used to determine the various geometric
design features of the roadway” (Ref. 1). When designing new or reconstructed roadways,
the design speed should always equal or exceed the proposed legal speed of the
roadway.
Table 2.1 (Ref. 7) shows the relationship of the functional classification, traffic data,
terrain, locale, and design speed to the various geometric design features listed on the chart.
It should be noted that there are situations when it will not be possible or reasonable to
meet the design standard for a particular feature in a given project. When this occurs, the
designer must bring this to the attention of the reviewing authority for approval of what is
being proposed, or suggestions on what other course of action to take. A design exception
must be approved by the reviewing authority when a substandard feature is allowed to
remain as part of the design. In this way, it can be documented that this was not an error
or oversight on the part of the designer and that every effort has been made to provide the
best design possible in the given situation