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Suggested Citation:"9. Software Implementation." 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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Page 49
Page 48
Suggested Citation:"9. Software Implementation." 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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Page 48

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

- 43 - 9. Software Implementation A computational engine with the proposed methodology has been developed and is available to HCQSC and Panel Members on request for beta-testing. The computational engine has been evaluated using a series of examples and use cases to ensure it replicates the methods accurately. The computational engine is not a commercial product expected to address all use cases that may arise; however, the chapter provides guidance for addressing a wide variety of use cases that may be encountered. Appendix A: Chapter 38 – Systems Analysis and the other report appendices contain a series of examples illustrating the application of the developed procedures. 10. Conclusions and Recommendations This project conducted research on the analysis of highway systems and produced a new chapter (Chapter 38-System Analysis) for the HCM 6th Edition. These new methods can be used to evaluate operations along networks that include both freeways and urban streets. The methods can also evaluate the impact of spillback into freeways and into urban streets from downstream facilities. In summary, the following were accomplished: Selection of appropriate performance measures: Travel time was selected as the performance measure to evaluate highway systems. Travel time measures are already used in the HCM to evaluate urban streets facilities. For freeway facilities, we developed additional models and methods to evaluate freeway performance by lane, as spillback affects each lane of the freeway differently. In addition, a trip-based performance measurement framework was developed to provide travel time estimates for given O-Ds within a highway network. O-D measures reflect traveler experience and are well aligned with recently available data collection methods which track individual trips. These new measures are intended to complement segment-based measures provided in other HCM chapters. Evaluation of queue spillback into freeways: Queue spillback into the freeway occurs due to insufficient capacity in at least one element of the off-ramp: either the ramp proper, or the downstream ramp terminal. The blockage of one or more freeway lanes adversely affects performance, and the extent of the blockage effects depend on various factors including the design of the facility, the cause of the blockage, and the length of the queue. Video and detector data from several locations were obtained, and used along with microsimulation to develop the methodological framework. The methodology developed is based on the calculation of demand and capacity at the downstream ramp terminal using the respective Interrupted Flow methods. It expands the Oversaturated Segment Evaluation for freeway facilities (HCM Chapter 25) and accounts for spillback and its effects by lane along the freeway mainline. Evaluation of queue spillback into urban streets: Queue spillback into urban streets occurs due to insufficient discharge capacity into the freeway merge. It may occur due to oversaturated conditions at the merge segment or the presence of ramp metering. Video and detector data from several locations were collected to understand how intersections are affected by on-ramp queue spillback. Microsimulation was used to complement the evaluation of signalized ramp terminals and to analyze systems with unsignalized intersections. The proposed methodology integrates the Interrupted Flow methodologies with the Freeway Facilities procedure to account for constraints of the on-ramp capacity. Several adjustments were developed to estimate the impacts of queue spillback from an on-ramp into upstream signalized and unsignalized intersections, including roundabouts. Lane by lane performance measures for freeways: Freeway speeds can vary widely depending on the lane used, and each O-D is likely to use a specific lane or set of lanes along each segment. Spillback affects

- 42 - Figure 36 – Origins and destinations for a sample network (Gainesville, FL) The methodology in Chapter 38 generates the following performance measures: Freeway Facilities: • Flow, free-flow speed, operating speed, and capacity for individual lanes • Expected travel speed along each segment and each lane Urban Streets Facilities: • Travel time along each segment • Expected travel speed along each segment System Analysis: • Total and free-flow travel times • Travel Time Index • Average travel speed

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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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