Best practices call for designers to select a design speed that is high enough so that most drivers will travel at or lower than the design speed, but low enough so that the physical effects of the design will be manageable and acceptable.

Selecting a Design Speed

Highway designers select a design speed, which is used to help establish the three-dimensional design features. The 2001 AASHTO Policy highlights the concept of choice through a new definition of design speed:

"Design speed is a selected speed used to determine the various geometric design features of the roadway. The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway."
AASHTO Policy on Geometric Design (2001)

Selection of an appropriate speed is left to the judgment of the designer, with general guidance provided by AASHTO as noted in Exhibit F-7 (CSD_154). Best practices call for designers to select a design speed that is high enough so that most drivers will travel at or lower than the design speed, but low enough so that the physical effects of the design (alignment, roadside, etc.) will be manageable and acceptable.

Exhibit F-7 (CSD_154)
Ranges of Design Speed Recommended by AASHTO
Design speed is the single most important choice designers make. The choice of a design speed should be made carefully, with full recognition of the context of the project. A good illustration of the effect of selecting a reasonable design speed is provided by a project performed by the Connecticut DOT in the town of Brooklyn. Selection of the initial design speed produced significantly greater requirements for longer vertical curves, and hence greater earthwork and right-of-way impacts. The resulting design was viewed as being overly impacting on the surrounding terrain. Moreover, the existing safety performance of the roadway did not indicate a problem related to the vertical alignment or sight distance. As a result, CTDOT revised the design, selecting a lower design speed, which produced an alignment considered to be substantively safe, with fewer impacts and lesser cost.

Traditional design practices and training of highway designers results in design speed being equated with design quality. In other words, many designers view a 60 mph highway as qualitatively better than a 50 mph highway. This view tends to be more valid in the rural environment, but even so, the substantive safety differences between the two are generally overestimated. It is certainly true that designs that support a higher speed have a greater margin of safety for faster drivers than other designs. Acceptance of a slightly lower design speed (say, from 60 mph to 55 mph) may, in some cases, result in an acceptable plan with no loss of substantive safety. An example of this is given by one of the case studies from Minnesota. Design for a slightly lower design speed than was originally envisioned enabled a suitable realignment of a highway and incorporation of enhancement features, without a serious degradation in the safety of operational efficiency of a highway and incorporation of enhancement features, without a serious degradation in the safety of operational efficiency of the highway.

Interestingly, all pilot state staff noted that speed consistency along a highway is as or more critical to good operations than the design speed. FHWA's IHSDM offers a new tool, a design consistency module, that allows the evaluation of expected speed behavior along a two-lane rural highway.

A challenge to context sensitive designers in the urban environment is to produce a high quality design where low speeds are considered to be safer. Conflicts with pedestrians, or immovable roadside objects (such as may exist in areas of limited right-of-way) call for lower speeds to achieve substantive safety. Indeed, European Context Sensitive Design practice as uncovered by an FHWA/AASHTO International Scanning Tour focuses on specific design actions intended to produce and maintain lower speeds through towns or developing areas. Referred to as traffic calming, treatments such as speed humps, diverters, chicanes, road narrowing, and other treatments represent best practices for low speed urban conditions where pedestrian safety and mobility is a primary concern.