The objective of this research was to develop material for the HCM that allows the analysis of corridors that include freeways and surface streets (e.g., Exhibit 2-1 of the HCM, 6th Edition). The research emphasized the use of field data (existing and new) for the development and validation of methodologies. The HCM material is based on the 6th edition of the HCM and includes:
- Performance and service measures for trips within corridors that use both the freeway and surface street facilities (urban streets for urban areas; multilane highways and two-lane highways for rural areas). All affected modes should be considered.
- An analysis framework, primarily for planning and design purposes, that can be used to estimate the performance measures for alternative designs (including active traffic and demand management and other operational strategies) based largely on 6th edition HCM methodologies.
- Adjustment factors and recommended modifications for HCM methodologies based on interactions between the freeway and surface street methodologies (e.g., queue spillback from ramp meters and ramp terminals, effects of platooned on-ramp vehicles).
- Guidance on scoping the analysis, including spatial and temporal boundaries for the analysis corridor.
- Limitations of the framework and methodologies. Conditions under which other analytical approaches would be more practical should be described. The capability of the framework and methodologies to accommodate demand shifts should be discussed.
- Data requirements for the use of the framework and methodologies, including techniques for collecting the data.
- Information on how traffic simulation, dynamic traffic assignment, and other alternative analysis approaches can complement the HCM material including comparing, integrating, and interpreting the analysis results.
- Guidance on incorporating probe vehicle, crowd-sourced data, and other emerging data sources into the generation of performance measures and validation of analysis results.
- Example problems illustrating use of the framework and methodologies.
TASKS
Phase I
Task 1. Critically review the existing HCM methods and identify weaknesses related to trips using freeways and surface streets.
Task 2. Critically review prospective performance measures including how they could be used and the data required to generate them. Propose a feasible framework for an HCM systems analysis based on feedback from the NCHRP 15-57 panel and the TRB Highway Capacity and Quality of Service (HCQS) Committee.
Task 3. Develop a preliminary analytical framework for HCM systems evaluation based on the results of Tasks 1 and 2.
Task 4. Develop a data collection plan for developing recommended improvements to the HCM and validating existing and new methodologies.
Task 5. Prepare an interim report documenting the entire effort during Phase I of the project. Meet with the project oversight panel to discuss the interim report.
Phase II
Task 6. Collect and assemble all data and develop and calibrate the draft methods prescribed in the Interim Report.
Task 7. The objectives of this task are to (a) evaluate the analysis methods developed under a variety of scenarios, (b) identify limitations of the methods, (c) evaluate the computational engine (Task 8), and (d) develop example problems and case studies.
Task 8. Develop a computational engine and accompanying documentation for the new methods developed and recommend adjustments to existing computational engines.
Task 9. Ensure that the methodology is adequately replicating conditions that are found in the field in terms of key performance measures.
Task 10. Develop a draft final report documenting the entire research effort and the recommended HCM procedures. The report will detail recommended modifications to existing HCM procedures and document the computational engine.
Task 11. Modify the draft final report based on comments received from the project oversight panel and the TRB HCQS Committee. The final deliverables will include (1) a final report that documents the entire research effort and includes draft modifications to the HCM as an appendix; (2) a computational engine and accompanying documentation for any new methodology and recommended adjustments to existing computational engines (the computational engine should conform to “Working Paper #6: Recommendations on Computational Engines”); and (3) a stand-alone technical memorandum titled “Implementation of Research Findings and Products”.
BACKGROUND
Highway agencies and municipalities are increasingly interested in corridor-level analysis, creating a need for accurate methods to measure and predict traffic operations across multiple facilities in a corridor. Recent editions of the Highway Capacity Manual (HCM) have improved analysts’ ability to consider the disparate elements of "Freeway Facilities" (Chapter 10) and "Urban Streets" (Chapter 16) in their respective system contexts, but the HCM is generally silent on the interactions between these two major facility classes. In fact, the structure of the HCM tacitly inhibits analyses of these interactions by using different performance and service measure approaches. This artificial separation does not reflect motorists’ daily experiences, which often involve transitioning from one facility type to another on any given trip. The lack of a methodology to analyze the operational characteristics of these transitions could result in designs that fail to address certain congestion problems.
Interactions between freeway and surface street facilities typically take place at interchanges. At the surface street end of a freeway off-ramp, an oversaturated intersection can cause queues that may back up to and affect operations on the freeway mainline. Conversely, congestion at the freeway end of an on-ramp can result in queues extending upstream towards the intersection that may affect operation on the surface street. These interactions are not explicitly accounted for in the HCM. Proper consideration of these effects is important in evaluating alternative designs, evaluating active transportation and demand management (ATDM) and other operational strategies, and estimating system performance measures such as travel time reliability. The upcoming 6th edition of the HCM begins to address ATDM and travel time reliability, though most of the material is for freeway facilities.
Many microscopic simulation software packages are designed to allow analysis of freeways and surface streets simultaneously, and the current state of the practice is to use simulation for such analysis. However, unlike many of the other elements of microscopic simulation, freeway--surface street interactions have no deterministic analog either in the literature, in general, or in the HCM, in particular, and thus lack the performance benchmark that the HCM is able to provide. Enhancing the ability of the HCM to address these corridors will provide analysts with another tool that for many situations will be easier to use.
