An interchange ramp is a roadway that connects two legs of an interchange. Ramp crosssection elements are discussed in Art. 2.3, Cross-Section Design. Elements contributing to horizontal and vertical alignments are designed similar to any roadway once the ramp design speed has been determined.
To design horizontal and vertical alignment features, a design speed must be determined for each ramp. Since the driver expects a speed adjustment on a ramp, the design speed may vary within the ramp limits. Table 2.28 includes three ranges of ramp design speeds that vary with the design speed of the mainline roadway. The ramp design speed range is determined by judgment based on several conditions:
• The types of roadways at each end of the ramp and their design speeds
• The length of the ramp
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TABLE 2.28 Guide for Selecting Ramp Design Speed
Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission. |
• The terminal conditions at each end
• The type of ramp (diamond, loop, or directional)
Diamond ramps normally have a high-speed condition at one end and an at-grade intersection with either a stop or a slow turn condition at the other. Upper — to middle — range design speeds in Table 2.28 are normal near the high-speed facility. Middle — to lower-range design speeds are usually used closer to the at-grade intersection. Loop ramps may have a high-speed condition at one end and either a slow — or a high-speed condition at the other. Loop ramps, because of their relatively short radius, usually have lower-range design speeds in the middle — and slow-speed end of the ramp, and upper — to middle-range design speeds nearer the high-speed terminal(s). Directional ramps generally have high-speed conditions at both ends. They are normally designed using an upper — range design speed, and the absolute minimum design speed should be from the middle range.
