This article provides information to assist the designer in determining lane widths, pavement cross slopes, shoulder widths, interchange cross-section elements, medians, curbs, pedestrian facilities, and grading and side slopes. The number of lanes for a given roadway facility is best determined using principles and procedures contained in the “Highway Capacity Manual” (Ref. 10). This manual analyzes roadways to determine an appropriate “level of service,” by which a letter value (A through F) is assigned depending on the volume of traffic and other geometric features. Table 2.20 provides a design guide for level of service for various facilities by functional classification and terrain or locale. The table includes a brief description of the characteristics of each level of service.
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TABLE 2.20 Guide for Selecting Design Service Level As Determined by Function and Terrain or Locale Minimum level of service for area and terrain or locale
A: Free flow, with low volumes and high speeds. B: Stable flow, speeds beginning to be restricted by traffic conditions. C: In stable flow zone, but most drivers are restricted in freedom to select own speed. D: Approaching unstable flow; drivers have little freedom to maneuver. E: Unstable flow; short stoppages may occur. F: Forced or breakdown flow. Source: Location and Design Manual, Vol. 1, Roadway Design, Ohio Department of Transportation, with permission. |
