BACKGROUND

The freeway weaving analysis methods in the 2000 Highway Capacity Manual (HCM) are based on research conducted from the early 1970s through the early 1980s. Subsequent research has shown that the methods' ability to predict the operation of the facility is limited. Other concerns about the methods include lack of consistency in application with other freeway methods, the difficulty of determining the service measure in the field, and the difficulty of comparing the analysis results with the results of simulation modeling.

OBJECTIVES

The objectives of this project were to develop improved methods for capacity and level-of-service analysis of freeway weaving sections and to develop a revised Chapter 24 of the 2000 HCM. Compatibility of the methods with those of other HCM freeway chapters, particularly the chapter on Ramp Junctions, were assessed.

STATUS

The results of the project are being incorporated into the Year 2010 edition of the Highway Capacity Manual by NCHRP Project 3-92. 

TASKS

Accomplishment of the project objectives will require at least the following tasks. (1.) Analyze, describe, and critique pertinent domestic and international literature on analysis of freeway weaving sections. Summarize information on the effectiveness of existing analytical methodologies for freeway weaving, emphasizing any available information on their compatibility with ramp junction methodologies. (2.) Describe alternative frameworks for a new weaving methodology. This could include the existing methodology, variations of the existing methodology, or new approaches such as lane-by-lane analysis. (3.) Catalog existing field-data sets (freeway weaving and ramp junctions) and assess their usability for this project. Some known sets that may be of value are as follows: NCHRP Project 3-55(5), FHWA Freeway Facilities, NCHRP Project 3-37, the NGSIM effort, and data from traffic management centers. (4.) Develop a field-data collection and analysis plan to augment the usable data sets identified in Task 3 for the most prevalent freeway weaving section configurations over the full range of operating conditions. The plan should identify the number and parameters of the sites where data will be collected (e.g., design configuration, volume ranges). A portion of the data should be reserved for validation purposes in Tasks 7 and 8. (5.) Within 6 months of the contract start date, submit an interim report on the information developed in Tasks 1 through 4. The interim report shall also contain a detailed, updated work plan and a revised budget for the remaining tasks. (6.) Execute the approved data collection plan. (7.) Assess the ability of the current Chapter 24 methodology to predict speed and level of service using field data. (8.) Develop an improved freeway weaving methodology that predicts capacity, speed, and a service measure (if it is not speed). Demonstrate the methodology's advantages in predicting speed and level of service using field data. The methodology should be clear and rational and the input data should be reasonably accessible to practitioners. (9.) Evaluate the consistency of the results of the proposed weaving and existing ramp junction analysis methodologies by analyzing configurations on the boundary between them (e.g., long auxiliary lanes, closely spaced on- and off-ramp segments). Propose a framework for ramp junction analysis that is compatible with the freeway weaving methodology developed in Task 8. Develop a scope of work and budget for revising the Freeway Concepts, Freeway Facilities, and Ramp Junctions chapters. (10.) Meet with the panel to discuss the results of Tasks 7, 8, and 9. (11.) Rewrite Chapter 24 suitable for inclusion in the HCM, including example problems and any necessary appendices. Identify other sections of the HCM affected by the changes in the chapter. (12.) Develop a computational engine in a convenient format that replicates the computations of the methodology developed in Task 8 and that can be used by software developers of highway capacity-related packages.
(13.) Submit a final report that documents the entire research effort and includes the Task 11 chapter as a stand-alone document and the Task 12 engine on a CD-ROM.