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Suggested Citation:"1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Highway Capacity Manual Methodologies for Corridors Involving Freeways and Surface Streets. Washington, DC: The National Academies Press. doi: 10.17226/25963.
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- 3 - 1. Introduction Background The procedures detailed in the 6th Edition of the Highway Capacity Manual (HCM) estimate capacity and several operational measures, including those determining Level of Service (LOS), for freeway facilities as well as surface streets. However, the HCM lacks a methodology for evaluating the operational performance of networks involving freeway-urban street interactions. The current HCM methods focus on operational analysis of individual segments or sections, which may not correspond to the traveler’s experience of an entire trip which traverses multiple different types of facilities. These methods do not consider cases in which spillback occurs from one type of facility to another, or across an entire origin- destination route, which is necessary for evaluation of network performance. Currently, the procedure for evaluating Signalized Intersections (HCM 2016 Chapters 19 and 31) predicts the average expected queue length at an approach given various combinations of geometric- or traffic-related inputs. Similarly, the Freeway Facilities procedure (HCM 2016 Chapters 10 and 25) estimates the maximum expected queue length at an on-ramp in the case of oversaturated conditions on the freeway mainline. However, the effects of these queues as they propagate upstream – onto a freeway mainline or a surface street intersection – are not considered. Research Objectives and Report Organization The objective of this research project is to develop materials for the HCM in order to modify the freeway analysis methods and the urban street methods so that the effects of operations from one facility to the other can be evaluated. This report summarizes the entire research effort and provides the proposed new HCM Chapter (Chapter 38) in Appendix A - Chapter 38 - System Analysis. Section 2 of this report provides the literature review conducted at the beginning of the project, while Section 3 summarizes the results of a critical review of existing HCM methods. Section 4 discusses the proposed performance measurement framework for the systems-based approach. Section 5 provides an overview of the methodology developed for evaluating queue spillback from off-ramps into freeways, including a review of the relevant data collection. Similarly, Section 6 discusses the data collection and procedures to evaluate queue spillback into urban streets. Section 7 presents the models developed to estimate lane-by-lane speeds and flows for freeway segments. Section 8 summarizes the methodology developed to estimate travel times for origin-destination pairs within freeway-urban streets networks. These travel times are estimated based on the research described in Sections 5, 6 and 7. Section 9 discusses the computational software engine which replicates the methods developed. Section 10 provides the project’s conclusions and recommendations. Several appendices are also provided which detail various aspects of the research: Appendix A - Chapter 38 - System Analysis (This appendix provides the new HCM Chapter 38, which is recommended for inclusion in the HCM 6th Edition.) Appendix B - Off-ramp Queue Spillback Check Appendix C - Off-ramp Queue Spillback Analysis Appendix D - On-ramp Queue Spillback Check Appendix E - On-Ramp Queue Spillback Analysis Appendix F – Freeway Facilities Lane-by-lane Analysis Appendix G - Glossary

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The procedures detailed in the 6th Edition of the Highway Capacity Manual (HCM) estimate capacity and several operational measures, including those determining Level of Service, for freeway facilities as well as surface streets.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 290: Highway Capacity Manual Methodologies for Corridors Involving Freeways and Surface Streets introduces materials to help modify the freeway analysis methods and the urban street methods so that the effects of operations from one facility to the other can be evaluated.

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