A critical examination is made of the conventional method for highway sizing, that is, determination of lane requirements. Ranked hourly traffic volume distributions, obtained from 1977 Kentucky volume stations are examined to test certain assumptions common to the conventional approach. Several of these distributions have no distinct "knee" (point or zone of sharper curvature) and, for those which do, the knee is most frequently found outside the normally anticipated range. Of equal importance, the knee location can be arbitrarily altered simply by changing the number of highest volume hours that are examined. The knee historically has been taken to represent a volume upon which to base capacity designs. The fundamental fallacy of the conventional procedure is its focus on a single design hour and its orientation toward conditions experienced by the highway rather than the user. This can readily be overcome by basing size decisions on an alternate criterion such as the percentage of vehicles that suffer congestion during the design life. An example demonstrating this concept is presented. More significant improvement can be achieved by directly computing the economic efficiency of investment in additional lanes. An example is presented to demonstrate current capabilites for such computations. The example also demonstrates that current procedures do not always yield the most economical designs and that the most economical highway size is affected by the specific shape of the traffic volume distribution. Use of economic efficiency analysis as a standard tool in evaluating critical sizing decisions is highly recommended.
- Record URL:
University of Kentucky, LexingtonCollege of Engineering, 176 Raymond Building
Lexington, KY United States 40506
Frankfort, KY United States 40622
Washington, DC United States 20590
- Crabtree, J D
- Publication Date: 1982-8
- Pagination: n.p.
- TRT Terms: Economics; Traffic distribution; Traffic lanes; Traffic volume
- Old TRIS Terms: Highway sizing
- Subject Areas: Design; Economics; Highways; Planning and Forecasting; I21: Planning of Transport Infrastructure;
- Accession Number: 00388817
- Record Type: Publication
- Report/Paper Numbers: UKTRP-82-13 Final Rpt.
- Contract Numbers: KY-80-83
- Files: TRIS, USDOT, STATEDOT
- Created Date: Sep 28 1984 12:00AM