Chapters 9 and 11 of the 1985 Highway Capacity Manual
(HCM) cover signalized intersections and arterial streets, respectively. Neither of these chapters, nor others in the HCM, address two important considerations: (1) signal coordination needs and progression characteristics of signalized ramp terminals and other closely spaced signalized intersections, and (2) queue spillback on the ramps and onto the crossroad. Thus, a methodology is needed to analyze signalized ramp terminals and adjacent intersections simultaneously. This methodology should be capable of addressing both coordinated pretimed and coordinated actuated signal systems.
The objective of this research was to develop and validate an appropriate methodology for determining capacity and level of service (LOS) at signalized ramp terminals, including adjacent closely spaced arterial intersections
The research consisted of the following tasks: (1) Conduct a state-of-the-art review to evaluate domestic and international experience on capacity and LOS analysis of interchanges and closely-spaced intersections. (2) Identify appropriate measures of effectiveness (MOEs) for the analysis of interchange ramp terminals. Recommend MOEs for use in the methodology to be developed in subsequent tasks and consider their compatibility with other chapters of the HCM, particularly Chapters 9 and 11. (3) Assess the applicability and validity of existing models for analysis, simulation, and optimization of interchange ramp terminals and at least one closely spaced signal on either side of the interchange. Identify the strengths and weaknesses of each model. Determine whether existing capacity procedures for signalized intersections adequately address interchanges such as Single Point Urban Interchanges (SPUIs) having only one signal on the arterial crossroad, or whether such interchanges need to be addressed by the proposed methodology. (4) Formulate several alternative methodologies to determine the recommended MOEs and to address the weaknesses identified in Task 3. Both analytical and simulation approaches should be explored. Assess the alternative methodologies and recommend the most promising methodologies for further development in subsequent tasks. Investigate the impacts of each alternative methodology on the HCM. Identify the type of software that may be needed to implement each alternative methodology. (5) Prepare an interim report presenting the results of Tasks 1-4, a preliminary data-collection and analysis plan for Task 6, proposed criteria for evaluating alternative methodologies as part of Task 7, and an updated work plan for all remaining tasks. (6) Collect and analyze data according to the approved plan. Field sites should include a representative range of geographical, design, and operational conditions. (7) Evaluate the alternative methodologies using field data. Select, refine, validate, and document the most promising methodology using the set-aside data. (8) Prepare a draft final report that contains the research results, a detailed explanation of the rationale for the selection of the recommended methodology, and a description of the software needed to implement the methodology. The report shall also recommend whether the proposed HCM material should appear as a new chapter or be incorporated into existing chapters. The final report shall also include an executive summary, which outlines the proposed changes to the HCM, and explains how they may affect other chapters.
: The revised final report was given to the contractor for NCHRP Project 3-55(6) and was used to develop Chapter 26, Interchange Ramp Terminals, of the HCM 2000.
The agency final report is available for viewing through this link: NCHRP Web Document 12: Capacity Analysis of Interchange Ramp Terminals: Final Report (1997)