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 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 examining.
The fundamental fallacy of the conventional procedure is its focus on a single design hour and its orientation toward conditions experienced by the 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 capabilities for such computations. the example also demonstrates that current procedures do not always yield the mos 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
