Design Speed

Design Speed

AASHTO defines design speed as follows:

Design speed is a selected speed used to determine the various geometric features of the roadway. The assumed design speed should be a logical one with respect to the topography, anticipated operating speed, the adjacent land use, and the functional classification of the highway.

Design speed is different from the other controlling criteria in that it is a design control, rather than a specific design element. In other words, the selected design speed establishes the range of design values for many of the other geometric elements of the highway (Figure 5). Because of its effect on so much of a highway’s design, the design speed is a fundamental and very important choice that a designer makes. The selected design speed should be high enough so that an appropriate regulatory speed limit will be less than or equal to it. Desirably, the speed at which drivers are operating comfortably will be close to the posted speed limit. 


In recognition of the wide range of site-specific conditions, constraints, and contexts that designers face, the adopted criteria allow a great deal of design flexibility by providing ranges of values for design speed (see Table 1). For most cases, the ranges provide adequate flexibility for designers to choose an appropriate design speed without the need for a design exception. A Guide for Achieving Flexibility in Highway Design (AASHTO) provides additional information on how to apply this flexibility for selecting appropriate design speeds for various roadway types and contexts.