2 Introduction

Rural highways account for a very significant portion of the national highway system and serve a vital mobility function and an increasingly important freight-movement function. These highways may connect larger municipalities but often travel through one or more small cities/towns. They are usually more varied in horizontal and vertical alignment than urban roadways. They may consist of combinations of two-lane highway, multilane highway, and occasional intersections (signals, roundabouts).

As urban areas continue to see growth further away from the central cities, rural highways in previously less developed areas are experiencing increases in traffic demand. Additionally, as urban area congestion continues to build, shipping companies are more frequently considering less congested two-lane and multilane highways in their routing decisions. The presence of commercial trucks on rural, particularly two-lane, highways pose additional challenges for maintaining acceptable levels of operational performance due to more variance in the geometric design of these facilities and less favorable passing opportunities.

Because of the extended lengths of rural highways and the variation in cross-section elements, it is very challenging to assess the operational quality of these highways across extended lengths (many miles) with the current analysis methodologies of the Highway Capacity Manual (HCM) (TRB, 2016). The results from NCHRP Project 17-65, Improved Analysis of Two-Lane Highway Capacity and Operational Performance (National Academies of Sciences, Engineering, and Medicine, 2018), advanced the capabilities of the HCM two-lane highway analysis methodology (Chapter 15). One major revision was the establishment of an analysis framework that was more amenable to a facility-level analysis approach than the previous analysis methodology. This framework provides for subdividing an extended length of two-lane highway into multiple contiguous segments of different types (passing constrained, passing zone, passing/climbing lane) that can be analyzed individually and collectively. However, there are still limitations in this methodology that need to be addressed to be able provide for a more comprehensive evaluation of rural highways, such as the impacts of intersections and short sections of multilane highway.

It is important to continue to extend the capabilities of the HCM analysis methodologies, particularly at the facility-level, so that roadway design and traffic engineers have the analysis tools they need for performing accurate and comprehensive facility evaluations. Furthermore, this is consistent with the fact that drivers typically evaluate the quality of their trip over its entire length, not just in separate segments.

When developing traffic analysis methodologies for the HCM, it is also important to consider the potential interaction between it and the preeminent roadway design reference, AASHTO’s A Policy on Geometric Design of Highways and Streets (aka the Green Book) (2018). With the recently released 7th edition of the Green Book, and its incorporation of new context classifications and discussion of the interrelationship of functional and context classifications, this project should take advantage of how these new Green Book features can inform the development of a rural highway traffic analysis methodology.

The objective of this research is to 1) develop reliability and quality of service predictive methodologies for rural road facilities accounting for the new context and functional classifications of the Green Book. The methodologies could be incorporated into the Green Book and into the HCM, and 2) develop a guidebook on application of the methodologies for a broad range of users. The resulting methods will lead to a rural highway facilities procedure suitable for incorporation into a future edition of the HCM.