The National Academies

NCHRP 07-26 [Active]

Update of Highway Capacity Manual: Merge, Diverge, and Weaving Methodologies

  Project Data
Funds: $400,000
Staff Responsibility: Dr. Waseem Dekelbab
Research Agency: Kittelson & Associates
Principal Investigator: Dr. Bastian Schroeder
Effective Date: 6/10/2019
Completion Date: 12/9/2021


Freeway congestion usually occurs at freeway merge, diverge, and weaving segments that have the potential to develop bottlenecks. To alleviate or mitigate the impacts of congestion at these segments, a number of active management operational strategies have been implemented such as ramp metering, hard shoulder running, managed lanes, etc. The current freeway merge and diverge methodologies in Chapter 14 of the 6th edition of the Highway Capacity Manual (HCM) were developed over 25 years ago using limited field collected data. Although weaving segment analysis was updated more recently, the relationship with the merge and diverge methodologies have not been clearly addressed. In addition to limited data, the methodology does not conform to the fundamental relationship of traffic flow, namely that flow is the product of speed and density. The HCM does not offer any methodology for lane drops or additions, which often occur in the vicinity of freeway merge/diverge segments. In the past decade, the data available to traffic engineers have expanded exponentially with ubiquitous sensor coverage of urban freeways and probe vehicle coverage of entire roadway networks.  These new datasets provide a wealth of information to support the development of updates or changes to the merge, diverge, and weaving segment methodologies, and potentially complement traditional data sources.



The objectives of this research are to (1) develop methodologies to update the HCM related to merge, diverge, and weaving methodologies; and (2) pilot the developed methodologies to demonstrate the full range of applicability of the proposed updates to the HCM. 

Accomplishment of the project objectives will require at least the following tasks.




PHASE I—Planning


Task 1. Conduct a literature review of relevant domestic and international research considered to be the state-of-the-practice on analysis of freeway merge, diverge, and weaving segments. The domestic review shall include research conducted through the NCHRP; FHWA; and other national, state, and pooled-fund sponsored research.


Task 2. Synthesize the literature review to identify the knowledge gaps and summarize information on the effectiveness of existing analytical methodologies for freeway merges and diverges, emphasizing any available information on their compatibility with basic freeway and weaving segment methodologies. These gaps should be addressed in the final product or in the recommended future research as budget permits. Discuss the quantity and quality of data required to achieve the project objectives.


Task 3. Propose methodologies, to be executed in Phase II, based on currently available data. At a minimum, the methodologies shall address the following:

a.    Data requirements and collections methods.

b.    Different geometric configurations, including those that have not been adequately described in the HCM (e.g., lane drops/additions, long acceleration/deceleration lanes for multilane ramps, auxiliary lanes, etc.).

c.     Commonly implemented freeway management tools and strategies (e.g., managed lanes, ramp metering, etc.).

d.    Compatibility with the basic segment methodology parameters (e.g., the free flow speed and capacity adjustment factors).


Task 4.  Identify areas of the HCM that require modification based on the proposed methodologies.


Task 5. Prepare Interim Report No. 1 that documents Tasks 1 through 4, includes the data archiving plan (see Special Note B), and provides an updated and refine work plan for the remainder of the research no later than 4 months after contract award. The updated plan must describe the process and rationale for the work proposed for Phases II though III.


PHASE II— Methodologies Development and Proposed HCM Update


Task 6. Execute Task 3 according to the approved Interim Report No.1, and submit a letter report at the end of Task 6 for panel review no later than 9 months after approval of Phase I.


Task 7. Develop the proposed update to the HCM with example problems including a comparison to the current HCM methodologies.


Task 8. Propose at least two states to pilot the proposed update to the HCM in order to test and demonstrate the full range of its applicability. The pilot includes presenting the proposed HCM update to the selected state department of transportation, collection required data to test the developed methodologies, performing the analyses, and presenting the results to and collecting feedback from the selected states. NCHRP must give approval of the state selection.


Task 9. Prepare Interim Report No. 2 that documents the results of Tasks 6 through 8 and provides an updated work plan for the remainder of the project. This report is due no later than 15 months after approval of Phase I. The updated plan must describe the work proposed for Phase III.


PHASE III—Pilots and Final Products


Task 10. Conduct the pilots and revise the HCM updates including the example problems after consideration of the feedback received from the two states.


Task 11. Prepare final deliverables including: (a) a final report that documents the entire research effort and (b) the proposed HCM updates with the example problems and implementation plan.


STATUS: Research in progress

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