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Road Management & Engineering Journal
Copyright © 1997 by TranSafety, Inc.
July 18, 1997
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Simulated On-the-Road Emergencies...
Planning and Designing Rail-Trails on Abandoned Rail Lines
Report Compares Methods of Analyzing Ramp-Free Junctions
Study Spotlights Railroad-Highway Grade Crossing Warning Systems

Report Compares Methods of Analyzing Ramp-Freeway Junctions

The 1994 Update of the Highway Capacity Manual (94HCM) included a new regression- based method for analyzing ramp-freeway junctions. An alternative to the method outlined in the 94HCM is the use of a microscopic simulation model such as FRESIM (FREeway SIMulator), also developed in 1994 by the Federal Highway Administration (FHWA). A thorough understanding of the benefits and limitations of both methods is advantageous in freeway analysis. To provide that advantage, the National Cooperative Highway Research Program (NCHRP) devoted its Report 385 to a discussion of both methods. NCHRP called their report "Comparison of the 1994 Highway Capacity Manual's Ramp Analysis Procedures and the FRESIM Model."

The method described in the 94HCM is a regression-based model for ramp-junctions. Possessing an extensive database, it provides valid results; but like many regression-based models, it can model only effects present in the database from which it is developed. The 94HCM model also has limitations for 8-lane freeways. A simulation model such as FRESIM, however, can compensate for those limitations. FRESIM models traffic explicitly, using sophisticated mathematical algorithms to model the complex driving behaviors of each individual vehicle. Nevertheless, FRESIM is not without limitations, notably its inaccuracy in predicting the merging process. For widespread use, FRESIM must also become more user-friendly.

One goal of the research documented in Report 385 was to make recommendations that would provide more consistency between the 94HCM and FRESIM models. Researchers noted, "In its present form, FRESIM is sufficiently at variance with the 94HCM in predicting operational conditions in the vicinity of ramp junctions to make it acceptable for use as an alternative for location-specific analyses." They also noted the 94HCM suffers from inconsistency in the variables of its algorithms. However, with correction and refinement of its problem areas, FRESIM could expand the data and help refine the algorithms of the 94HCM. As such, FRESIM could be a significant tool in allowing "a more consistent and thorough calibration of regression models for the subsequent editions of the Highway Capacity Manual."

Copyright © 1997 by TranSafety, Inc.

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