Improved Analysis of Two-Lane Highway Capacity and Operational Performance

Abstract

Two-lane highways account for a very significant portion of the national highway system and serve an essential function for the movement of people and goods. The Highway Capacity Manual (HCM) contains a chapter that provides an analysis methodology for two-lane highways. Unfortunately, the current HCM analysis procedure has been criticized on several issues, such as the speed-flow relationship, appropriate service measures, treatment of large trucks, guidance on base free-flow speed estimation, accuracy of passing lane adjustments, and limitations in analysis scope. This project sought to address these limitations and gaps. Accomplishments from this project include the following: the development of a more realistic speed-flow relationship; the introduction of a new service measure—follower density; a new headway threshold value to better identify follower status; development of a percent followers-flow relationship; elimination of passenger car equivalent (PCE) values and direct use of percentage of heavy vehicles in the models for performance measure estimation; the inclusion of a quantitative adjustment based on posted speed limit for the estimation of base free-flow speed (BFFS); the development of new functions for passing lanes—effective and optimal lengths and performance measure improvements for 2+1 sections; and the development of a method for combining the analysis of multiple contiguous segments into a facility-level analysis. This project also introduced features to improve the ease of use of the methodology in the HCM, such as elimination of tables requiring interpolation, treating trucks explicitly instead of through PCE values, using a single service measure, and eliminating the PTSF measure. And finally, two modern microsimulation tools were identified that are capable of accurately modeling two-lane highways: SwashSim and TransModeler.