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Suggested Citation:"10. Conclusions and Recommendations." 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 50
Suggested Citation:"10. Conclusions and Recommendations." 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 50

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- 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

- 44 - each freeway lane differently, and its effects depend on the site design and the length of the queue. This project developed models that estimate speeds and flows by lane. Detector data were collected from a variety of locations around the US, and analytical models were developed to predict the lane flow distribution and lane speed. These models considered a variety of factors including v/c ratio, presence of nearby ramps, heavy vehicle percent, and grade. Regression models built from the field data demonstrated that FFS and capacity values for each lane can be obtained as a percentage of the segment average with satisfactory results. Development of an O-D analysis framework: A new methodology was developed to estimate travel times by O-D within a highway system. This methodology combines the tools of several HCM chapters within the Uninterrupted Flow and Interrupted Flow volumes. It also builds on the research previously described to evaluate queue spillback and system effects The limitations of the methods include the following: Queue spillback into freeways: • Overlapping bottlenecks cannot be addressed as they require iterative processes; • Managed lanes are not addressed by the methodology. Queue spillback into urban streets: • Roundabouts with two lanes cannot be addressed; • The HCM methodology for roundabouts does not provide a multi-period analysis procedure. Therefore, the delay results for the proposed methodology may not be accurate when there is a queue at the start of the analysis period. • The procedures do not specifically address alternative intersection designs, such as SPUI, DDI; however, the analyst may estimate capacities and queue lengths associated with a variety of facilities and implement the methods described here for a variety of configurations; • Indirect impacts of queue spillback, such as turn bay overflow are not addressed by this methodology; • The procedure relies on accurate estimates of the capacity of the merge obtained using the Oversaturated Segment Evaluation (HCM Chapter 25); however the estimation of this capacity is listed as a limitation of Chapter 25. Freeway lane by lane analysis: • Flow distribution models were not developed for segments with 5+ lanes. For these cases, using equal distributions of flows and speeds are recommended. System analysis: • HCM Travel time reliability methodologies for Freeways (Chapter 11) and Urban Streets (Chapter 17) have significant differences in their procedures. Therefore this project made no attempt to incorporate reliability analysis in the new procedures. In the future, it would be useful to develop a LOS framework for the travel time performance measures yielded by this methodology to evaluate highway systems. Similar to other performance measures used in the HCM, communicating the values of performance measures to a lay audience may be challenging, while LOS may be a more useful construct.

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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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