Determination of DDHV from Directional Traffic Flows

The traditional two-way design-hour volume (DHV) is multiplied with directional split (D) to obtain the value of directional design-hour volume (DDHV) for multilane highways. A number of previous studies have revealed some limitations of this method. It has been found that the approach of deriving the DDHV from the two-way design hourly volume could fail in appropriately recognizing the peaking characteristics of traffic in the two individual directions of traffic flow. In addition, by applying the traditional approach, even in the cases of permanent traffic counter (PTC) sites at which data are available for the whole year, an analyst could underestimate or overestimate the value of DDHV. Such limitations of the traditional approach have been attributed mainly to the highly variable nature of directional split. Presented in this paper is an approach of determining DDHV from directional flows rather than using a directional-split value to derive DDHV from DHV. Highway-traffic data from PTC sites on Alberta, Canada, and Minnesota highway networks are used to describe the proposed methodology in which the two directional flows at a site are treated as two different traffic streams. One-way highest hourly volumes are ranked to obtain the required DDHV value without the application of directional split. The main advantage of the suggested approach is that it does not require the use of the directional-split variable, which can be a major source of error in the estimation of DDHV. Application of the proposed approach is particularly suitable for operational analysis of multilane highways.