Design speed is defined by the AASHTO Green Book as: “the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.”
All geometric design elements of the highway are affected in some way by the selected design speed. Some roadway design elements are related directly to and vary appreciably with design speed. These include horizontal curvature, super elevation, sight distance, and gradient. Other elements are less related to design speed, such as pavement and shoulder width and clearances to walls and traffic barriers. The design of these features can, however, affect vehicle operating speeds significantly. As a result, more stringent criteria for these features are generally recommended for highways with higher design speeds. Conversely, less stringent criteria for these features may be more appropriate on roadways with lower design speeds. The selection of a particular design speed is influenced by the following:
Typically, an arterial highway warrants a higher design speed than a local road; a highway located in level terrain warrants a higher design speed than one in mountainous terrain; a highway in a rural area warrants a higher design speed than one in an urban area; and a high-volume highway warrants a higher design speed than one carrying low traffic volumes. Lower design speeds are appropriate for city streets. Higher volume rural highway facilities should use higher design speeds.
Most States and localities have adopted a range of acceptable design speeds for each of the major classes of highways and streets (i.e., freeway, other arterial, collector, and local). Designers need to balance the advantages of a higher vehicle operating speed gained through the use of a higher design speed against the flexibility lost in design. It may be more important to retain the maximum possible flexibility, so that a context-sensitive roadway that is more in tune with the needs of a community is designed using a lower design speed. As used here, the term “context sensitive” refers primarily to the land use and environmental conditions adjacent to the highway. For example, for any particular highway other than a freeway or major arterial, as land use density increases, the design speed would typically decrease. The design speed of an urban collector street passing through a residential neighborhood should be appreciably lower than that for a rural highway with the same functional classification. This also recognizes the fact that bicycles and pedestrians would be more likely to use a route located in an urban area.
Similarly, in areas that have significant historic interest or visual quality, a lower design speed may be appropriate in recognition of lower average operating speeds and the need to avoid affecting these historic or aesthetic resources. The Green Book agrees with this philosophy: Above-minimum design values should be used where feasible, but in view of the numerous constraints often encountered, practical values should be recognized and used. Along arterial streets, the controlling factor of design speed applies to a lesser degree than on rural highways or high-type urban facilities, such as freeways or expressways. On many of the arterial streets located in large urban areas, maximum vehicle operating speeds for several hours of the day may be limited to those at which the recurring peak period traffic volumes can be accommodated. Thus, speeds may be governed by the presence of other vehicles traveling en masse both in and across the through travel lanes and by traffic control devices, rather than by the physical characteristics of the street. During off-peak periods of low-to-moderate traffic demand, vehicle operating speeds are governed by such factors as speed limits, mid-block turns, intersection turns, number of driveways and entrances, traffic signal spacing, and signal timing. As a result, when arterial street improvements are being planned, the selection of the appropriate design speed must be balanced against such factors as speed limits, physical and economic constraints, and the probable running speeds that can be attained during off-peak hours. Although most States have adopted a range of allowable design speeds appropriate for each of the various functional classifications for use in the design of new or reconstructed highway facilities, situations may arise where even the use of the lowest typically acceptable value would result in unacceptably high construction or right-of-way costs or unacceptable impact on adjacent properties. In such instances, the design exception process may be employed. For example, the reconstruction of a two-lane rural arterial route through a relatively flat but environmentally sensitive area might need to employ a design speed of 80 km/h (50 mph) rather than the recommended value for this functional classification of 100 km/h (60 mph).
Source: Flexibility in Highway Design pp. 56-59
Published: 1997
