• Overview of alternative analysis methods, to the HCM7, for bicycles and recommendations for future bicycle operations’ research needs.

With the heterogeneity of cross-section (i.e., roadway segment type) composition over such distances and the disparate HCM7 service measures (density, follower density, delay) across these segment types, the process for performing an HCM7 facility analysis across the variety of contiguous segments contained within a rural highway facility is not necessarily straightforward. This Guide proposes an analysis framework for assessing the level of service (LOS) of automobiles on a long rural highway facility (20+ miles). In addition to LOS, several other facility-level performance measures are presented along with discussion about the analysis context in which such measures are useful for evaluating overall traffic operations along the route.

While simulation is always an option for analyzing a stretch of rural highways, the level of effort would be high for typical rural highway distances considered for analysis. In some situations, simulation may be warranted, but the methodology described in this Guide would still be a good first step and may even be sufficient. This methodology would also be much more efficient for performing “what-if ” scenario testing, where relative differences in results are the primary concern.

Reliability analyses on freeways and arterials are typically based on HCM7 guidance on scenario generation and predictive reliability. However, because of the typically limited data availability on rural highways, the reliability analysis in this report is focused on historical probe vehicle data and meant to be used in conjunction with the HCM7 automobile LOS methodology.

Demand for cycling in urban areas and on rural highways is on the increase, yet it is not clear which analysis procedures are best suited to cover large rural highway facilities or statewide analyses. This Guide summarizes existing HCM7 analysis methods used for the bicycle mode as well as two popular alternatives: Level of Stress and Bicycle Compatibility Index. In addition, it proposes recommendations for future bicycle operations research needs based on two qualitative surveys.

This Guide is intended to serve as a companion to the HCM.

1.2 Guide Scope and Limitations

The facility-level analysis is important in assessing current conditions along important corridors for people and goods movement. The analysis methodology is also useful for evaluating facility performance for situations where significant changes in traffic demand or capacity may occur, such as in the following scenarios:

• Evaluation of a rural highway route to handle potential evacuation traffic demand (e.g., forest fire, hurricane).
• Evaluation of a rural highway route to handle potential diversion traffic demand due to an alternative route being closed or restricted due to construction, an incident (e.g., truck rollover), or a natural disaster (e.g., landslide/avalanche, flooding).
• Evaluation of a rural highway route to handle short-term spikes in traffic demand due to recreational activities (e.g., weekend ski season, Labor Day weekend beach travel, concert/festival).
• Evaluation of a rural highway route to handle short-term spikes in traffic demand and/or heavy vehicles due to season-specific activities (e.g., crop harvesting in farming regions).
• Evaluation of a rural highway route to handle a large increase in traffic demand as projected to occur as part of construction of a large generator for regional economic development (e.g., tribal casino, Amazon distribution warehouse).

The parameters of the scope for this project generally required that the developed LOS evaluation methodology make use of the existing analysis methodologies within the HCM. However,

to facilitate the development of a facility-level evaluation methodology, it was necessary to develop a few new computational procedures, largely to connect component pieces of highway into a single facility for evaluation purposes. Furthermore, some planning-level simplifications, such as the classification of terrain and the treatment of signal progression along an arterial, were implemented. Such simplifications were included to (1) reduce the segmentation process effort and/or (2) reduce the complexity of the calculation process where the return on such precision is minimized for the relatively long lengths of rural highway.

This Guide also introduces a method for evaluating rural highway operations with the use of probe vehicle data. Over the last decade, the spatial and temporal coverage of probe vehicle data available from third-party vendors has improved immensely. Many state agencies now pay for subscriptions to providers of such data and are making use of the data to supplement their traditional data sources (e.g., fixed-point sensors) for assessing and managing traffic operations on their roadways.

These data generally consist of average travel times/speeds and correspond to a sample of the vehicles traveling along a given roadway segment; thus, an important limitation of this data source is that it does not include flow rate. To obtain flow rates, agencies may supplement the probe vehicle data with fixed-point sensors or conduct field data collection using portable sensors on a regular basis. Another limitation of probe data is that their accuracy drops in low-traffic conditions, such as late night or early morning when fewer samples are available.

The spatial resolution of the probe data might also pose a challenge, particularly when analyzing very short segments. Probe data are typically reported for predefined segments of roadway—often referred to as traffic message channels (TMCs). The TMCs’ length usually ranges from approximately 0.6 to 2 miles, and the TMC boundaries do not necessarily match those used for other traffic analysis purposes, such as for segments as defined by the HCM. The temporal resolution of the measurements ranges from approximately 1 to 5 minutes. Currently, the quantity and quality of probe vehicle data are much greater for urban areas than for rural areas. This gap, however, will continue to narrow with time.

The automobile LOS methodology presented in this Guide is not intended to handle oversaturated traffic flow conditions. For multilane and two-lane highway segments, the HCM7 analysis methodologies do not include any mechanism to deal with traffic demand exceeding capacity. In some instances, short periods of demand exceeding capacity can be accounted for in the intersection analysis methodologies. The HCM7 should be consulted for further information on this topic.

1.3 Guide Organization

This Guide is organized into three parts.

Part I focuses on analysis methodology descriptions and consists of the following chapters:

• Chapter 1—Introduction. This chapter provides an overview of the Guide’s purpose, scope, and limitations. It also discusses the format of the Guide and how the user community can contribute to the content. Further, the chapter provides a summary of the research behind this Guide.
• Chapter 2—Rural Highway LOS for Automobiles. This chapter provides the methodology used to assess traffic operational quality and LOS for the automobile mode on rural highways.
• Chapter 3—Automobile Travel Time Reliability. This chapter describes methods for quantifying travel time reliability, from a historical perspective, based on probe vehicle travel speed measurements.

• Chapter 4—Bicycle Operations Analysis on Rural Highways. This chapter provides an overview of commonly used methods for assessing bicycle operations, recommendations for which methods are most appropriate for certain bicycle analysis situations, and recommendations for enhancements to the commonly used analysis methods.

Part II consists of Chapters 5 through 12. It provides an overview of the component HCM7 analysis methodologies that are incorporated in the rural highway analysis methodology for automobiles. This part does not replicate the full content of the relevant HCM7 analysis methodologies but rather summarizes the chapters and sections that are used within the rural highway analysis methodology. This material will be updated as necessary to reflect updates to the HCM.

Part III consists of Chapters 13 through 19. It focuses on case studies using real-world routes to demonstrate the analysis methodologies in the Guide. The material is contained in a separate part of the report to facilitate the inclusion of additional case studies in the future.

1.4 Supporting Resources and Tools

Several complementary resources are provided with this Guide.

LOS Calculation Software and Case Study Input Files. The LOS calculation methodology described in this Guide is available in the software tool HCM-CALC. This program can be downloaded from https://github.com/swash17/HCM-CALC. The Computational Engine chapter also provides an overview of the software tool. Input data files for the case studies for the HCM-CALC software are also available.

Scripting Code/Tools for “Reliability” Calculations/Output. Scripts, written in Python programming code (https://www.python.org/about/), to process probe vehicle data and produce a variety of visualizations are provided. More information is provided in Chapter 3.

KML Files for Case Studies. For each of the case studies, supporting information for the segmentation process is included in a KML file. [A Keyhole Markup Language (KML) file contains geographic and supporting data for use with geographic software visualization tools. More information can be found at https://en.wikipedia.org/wiki/Keyhole_Markup_Language.] Detailed information about the KML files is provided in the introduction to Part 3: Case Studies.

More information about these resources is contained in Chapter 15.

This Guide was developed through NCHRP Project 08-135: “Reliability and Quality of Service Evaluation Methods for Rural Highways.” A conduct of research report was also produced for this project, published as NCHRP Web-Only Document 392: Developing a Guide for Rural Highways: Reliability and Quality of Service Evaluation Methods (Washburn et al. 2024). That report contains additional details about the development of the material contained in this Guide.