The National Academies

NCHRP 08-135 [Completed]

Reliability and Quality of Service Evaluation Methods for Rural Highways

  Project Data
Funds: $400,000
Research Agency: University of Florida
Principal Investigator: Scott Washburn
Effective Date: 12/8/2020
Completion Date: 9/8/2023



Rural highways are a major part of the nation’s transportation system. According to Federal Highway Administration (https://www.fhwa.dot.gov/policyinformation/statistics/2016/), 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-1: Incorporation 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 Streets, 7th 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 were 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.



Task 1. Review literature and current practices. Review relevant literature and current practices from domestic and international sources. The product of this task shall be a technical memorandum summarizing relevant literature and current practice and identifying gaps therein.  

Task 2. Develop proposed predictive methodologies and associated data collection plans.

Task 2a. Develop proposed predictive methodologies. Develop reliability and quality of service predictive methodologies for rural highway facilities, considering all travel modes. Determine how these methodologies could be applied to rural corridors between various origins and destinations. Describe the expected inputs, outputs, strengths, and limitations of the methodologies, including trade-offs between through/local traffic and between travel modes. Determine how to incorporate each corridor component and its performance measure into the methodologies. These components should include, but not be limited to, rural two-lane segments, multilane segments, signalized intersection influence areas, and roundabouts. The new functional and context classifications in the Green Book must be incorporated. The product of this task shall be a recommended list of candidate methodologies for predicting reliability and quality of service for rural highways.

Task 2b. Develop data collection plans. Develop plans to guide the collection of data for rural and rural town contexts. These plans shall include extensive, high-resolution travel time data at the facility and trajectory levels, along with data regarding incidents, incident response and clearance times, work zone and maintenance activities, weather, and demand variation. Data should cover a 3- to 5-year timeframe to ensure that an adequate range and combination of data elements are captured. The research team shall develop (a) a list of multiple study locations that sufficiently covers different rural traffic scenarios and geographies and (b) the key parameters for data collection and the approaches or sources to be adopted to collect data. Candidate methodologies to be tested and the associated data collection plans shall be summarized in a technical memorandum and presented at an interim meeting. The methodologies developed in Task 2a and the data collection plans developed in Task 2b must be approved by NCHRP before Task 3 work begins.

Task 3. Collect data and analyze preliminary results. Implement the approved data collection plans developed in Task 2b. Analyze preliminary results against the proposed methodologies from Task 2a and determine any changes needed thereto. The product of this task shall be a technical memorandum summarizing data collection efforts and preliminary results. This memorandum must be approved by NCHRP before work on Task 4 begins.

Task 4. Refine predictive methodologies. Perform any methodology refinement needs identified in Task 3. Incorporate refined methodologies into the final report developed in Task 5. The table of contents for the user guidebook shall be submitted before the final report.

Task 5. Submit final reportSummarize the work performed on the previous tasks. The report shall include language regarding predictive methodologies for rural highway facilities suitable for potential inclusion in a future update of the HCM. Provide the full text of the user guidebook mentioned in Task 4 as a standalone document.


Status: Research is complete; publication is pending. 

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