Reliability and Quality of Service Evaluation Methods for Rural Highways

NCHRP project 08-135


Rural highways are a major part of the nation’s transportation system. According to Federal Highway Administration, rural highways comprise around 70% of all highway mileage. Rural highways often go through small communities with a variety of conditions, including adjacent land use context, roadway characteristics, and traffic control. The same rural highways provide for through and local traffic movements. The perception of quality of service of a rural highway can also vary, depending on the user and purpose. Hence, providing a multimodal, facility-based evaluation methodology, which currently does not exist, is of interest to state departments of transportation (DOTs).

A limitation in the Highway Capacity Manual6th Edition (HCM), is the facility-level analysis of rural roads. The HCM contains procedural analysis techniques for uninterrupted flow two-lane and multilane segments, but it does not contain a technique to analyze the capacity and level of service for rural highways with different segment types at the facility level. The HCM also contains facility analysis techniques for other roadway types, including interrupted flow urban streets and freeways, but not for rural highway facilities. Future research on two-lane highways is needed to build upon the work reported in NCHRP Web-Only Document 255: Improved Analysis on Two-Lane Highway Capacity and Operational Performance. Given that the HCM is accepted nationwide as the primary source on highway capacity and quality of service, the lack of a technical approach to address domestic rural highways is a major limitation.

Another limitation of the current HCM methodology for rural highways is the analysis horizon, which is limited to a single study period. Recently, the HCM incorporated a methodology to evaluate travel time reliability for freeways and urban streets (Chapters 36 and 37), through the work described in SHRP 2 Report S2-L08-RW-1Incorporation of Travel Time Reliability into the Highway Capacity Manual. With this approach, the time analysis horizon is expanded to several weeks or months to evaluate the variability and the quality of service that the facility provides to its users. Use of a distribution of level of service values mimics the variability of traffic conditions on the facility and provides a better understanding of the quality of service across time. By having more appropriate performance measures for these types of facilities, state DOTs can better allocate their scarce resources.

Yet another limitation of the current HCM methodology for rural highways is the limited consideration of quality of service for non-motorized users, which decreases with increased width from new general purpose lanes, from new turning lanes, and from increased turning radii.

In parallel, the AASHTO Policy on Geometric Design of Highways and Streets7th Edition (hereafter Green Book), has introduced the consideration of context classifications as an element of the geometric design process. The two new context classes supplement, but do not replace, the current functional classification system. The rural class applies to roads in rural areas that are not within a developed community, while the rural town class applies to roads located in developed communities. The classification results from NCHRP Report 855: An Expanded Functional Classification System for Highways and Streets. Given the known relationship between geometric design features and traffic operations, incorporating the Green Book’s context classification into the HCM for highway capacity analyses and design is needed.


The objectives of this research are the following:

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.

2.    Develop a guidebook on application of the methodologies for a broad range of users.


  • Technical memorandum summarizing relevant literature on cycling on rural highways and current practice and identifying gaps therein.  
  • Lead the development of proposed predictive methodologies for bicycles. This effort would consist of designing, administering and analyzing two online surveys, one for bicyclists and one for State DOTs.