The National Academies

NCHRP 03-37(2) [Final]

Capacity and Level of Service at Ramp-Freeway Junctions (Phase II)

  Project Data
Funds: $150,000
Research Agency: Polytechnic University
Principal Investigator: Roger P. Roess
Effective Date: 9/1/1994
Completion Date: 4/30/1996

The first phase of NCHRP Project 3-37, Capacity and Level of Service at Ramp-Freeway Junctions, resulted in the formulation of new regression-based models for the prediction of capacity and operating conditions in the vicinity of ramp-freeway junctions. These models were incorporated into the latest Chapter 5 of the Highway Capacity Manual (HCM). In a parallel effort, the Federal Highway Administration developed its latest freeway simulator, FRESIM, which will be released through McTrans for general use. Since the simulator can also be used to generate operational descriptions of ramp-merge and ramp-diverge areas, there is overlap in the potential use, application, an interpretation of the results of the two models.

The general objective of this research was to identify and investigate (1) common ranges of application, as well as ranges where one or the other model might be inappropriate for use; (2) consistency of internal logic used in each model; (3) consistency of results when models are properly applied to the same case; (4) comparative sensitivities of the models to key input variables; and (5) potential modifications to models, which would improve the consistency of results where both models are properly applied.

Accomplishment of this objective required the following tasks. (1) Identify common application areas where both FRESIM and the HCM methodology are legitimate problem-solving approaches through a comparison of each model's theoretical basis. (2) Identify, from the existing Phase I data collection locations, candidate sites for comparison, augmented as necessary by hypothetical data. (3) Identify and analyze specific scenarios that illustrate similarities and differences between the HCM and FRESIM models considering such factors as definition of terms, model structure, intrinsic relationships, calibration of parameters, and valid range of operation. (4) Based on the results of the previous tasks prepare the following: guidelines for the preparation of input data to minimize discrepancies that may be found between FRESIM, the HCM methodology, and field data; guidelines for the selection of the most appropriate technique, or combination of techniques, to maximize the effort and utility of the results in specific situations; recommendations for modification to both techniques to improve their accuracy and applicability; and recommendations on how to integrate analytical, empirical, and simulation models for future capacity and level-of-service methods.

Status: The revised final report was published as NCHRP Report 385, "Comparison of the 1994 Highway Capacity Manual's Ramp Analysis Procedures and the FRESIM Model."

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