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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

Highway Capacity Manual Methodologies for Corridors Involving Freeways and Surface Streets

The objective of this research project is to develop materials for the Highway Capacity Manual (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. The new methods produced 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. Travel time was selected as the performance measure to evaluate highway systems. For freeway facilities, the authors 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. These new measures are intended to complement segment-based measures provided in other HCM chapters. To evaluate queue spillback into freeways, 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. To evaluate queue spillback into urban streets, 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. The project developed models that estimate speeds and flows by lane. Detector data were collected from a variety of locations, 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. Finally, 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. Limitations of the methods and directions for the future are discussed.  

Language

  • English

Media Info

  • Media Type: Digital/other
  • Edition: Contractor's Final Report
  • Features: Appendices; Figures; References; Tables;
  • Pagination: 390p
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 01756658
  • Record Type: Publication
  • ISBN: 9780309684262
  • Report/Paper Numbers: NCHRP Project 15-57
  • Files: TRIS, TRB, ATRI
  • Created Date: Oct 29 2020 12:57PM