Safe and efficient two-lane highway service is a vital component of the nation’s transportation system. Many two-lane highways serve as the primary means for rural access to urban areas and the Interstate Highway System. Two-lane highways are a unique element of the surface transportation system because, in many instances, they serve extreme conditions of access and mobility often with very little intervention from traffic control devices. Development needs on the urban fringe and rural areas can degrade two-lane highway service. Coupled with the high costs and impacts associated with widening to four-lane highways or freeways, the nation increasingly relies on these two-lane highways to serve complex combinations of transportation needs including accessing adjacent land, commuting, multi-modal accommodation, and freight movement. In essence, designers and decision makers need the ability to analyze two-lane highway service in complex circumstances to evaluate two-lane highway capacity and operational capabilities to meet diverse traveler and freight movement needs. However, current tools to evaluate existing and proposed designs are inadequate for the task.
The Highway Capacity Manual (HCM) methodology is difficult to use for an established network. The capacity and operational performance analysis methods and models for two-lane highways are dated and limited to homogenous segments that do not account for the variations in route operational characteristics (development, transition zone, signalized intersection(s), alignment, speed, volume, etc.). Furthermore, collecting sufficient amounts of two-lane highway field data to use exclusively for the development of an analysis methodology is not practical. Simulation tools may need to be utilized. The TWOPAS simulation program, on which the current HCM methodology for two-lane analysis is based, is no longer supported. In addition, current measures of effectiveness are difficult to field measure.
The objective of this research was to (1) develop performance measures for operational and capacity analyses of two-lane highways and develop models to produce these performance measures in a HCM context, and (2) develop or modify a simulation-based analysis method for two-lane highways and offer guidance for when to apply a simulation versus HCM methods. The resulting methods should lead towards a two-lane highway facilities procedure suitable for incorporation into future editions of the HCM.
The research agency’s final report that documents the entire research effort is available as NCHRP Web-Only Document 255: Improved Analysis of Two-Lane Highway Capacity and Operational Performance at: https://www.trb.org/main/blurbs/177835.aspx
Note: Appendix G will replace Chapter 15 of the Highway Capacity Manual, and therefore, it is not available at this time.
