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Unformatted text preview: HIGHWAY DESIGN MANUAL 100-1 June 26, 2006 CHAPTER 100 BASIC DESIGN POLICIES Topic 101 - Design Speed Index 101.1 - Selection of Design Speed Design speed is defined as: "a speed selected to establish specific minimum geometric design elements for a particular section of highway". These design elements include vertical and horizontal alignment, and sight distance. Other features such as widths of pavement and shoulders, horizontal clearances, etc., are generally not directly related to design speed. The choice of design speed is influenced principally by the character of terrain, economic considerations, environmental factors, type and anticipated volume of traffic, functional classifica- tion of the highway, and whether the area is rural or urban. A highway in level or rolling terrain justifies a higher design speed than one in mountainous terrain. As discussed under Topic 109, scenic values are also a consideration in the selection of a design speed. In addition, the selected design speed should be consistent with the speeds that are likely to be expected on a given highway facility. Drivers adjust their speed based on their perception of the physical limitations of the highway and its traffic. Where a reason for limiting speed is obvious to approaching drivers, they are more apt to accept a lower design speed than where there is no apparent reason for it. A highway carrying a large volume of traffic may justify a higher design speed than a less important facility in similar topography, particularly where the savings in vehicle operation and other costs are sufficient to offset the increased cost of right of way and construction. A lower design speed, however, should not be assumed for a secondary road where the topography is such that drivers are likely to travel at high speeds. Subject to the above considerations, as high a design speed as feasible should be used. It is preferable that the design speed for any section of highway be a constant value. However, during the detailed design phase of a project, special situations may arise in which engineering, economic, environmental, or other considerations make it impractical to provide the minimum elements established by the design speed. The most likely examples are partial or brief horizontal sight distance restrictions, such as those imposed by bridge rails, bridge columns, retaining walls, noise barriers, cut slopes, and median barriers. The cost to correct such restrictions may not be justified. Technically, this will result in a reduction in the effective design speed at the location in question. Such technical reductions in design speed shall be discussed with and documented as required by the Design Coordinator....
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- Fall '08