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3 Chapter One INTRODUCTION AND OVERVIEW January 1993, He University of Idaho, in cooperation with Kittelson and Associates, Ruhr University, and Queensland Un~versi~ of Technology, inflated work on a project to develop new capacity and level of service analysis procedures for unsignalized intersections. This project, funded through He National Cooperative Highway Research Program (NCHRP) and designated project 346, is one of several research projects idendhed by the Transportation Research Board's (TRB) Committee on Highway Capacity and Qualm of Service (Committee A3AlO) needed to improve He capacity and level of service analysis techniques available to practicing traffic eng nears. It Restart of an effort by the Committee to develop a new version of He Highway Capacity Manual (HCM) by He year 2000. While most over chapters of He 1985 HCM are support by one or more nationally funded research projects, Chapter Ten borrows heavily from a procedure for two-way stop-controlled (TWSC) intersections originally developed in Germany. Unfortunately, He procedure was backed by only limited calibration data reflecting conditions found in He United States. For all- way sto~con~olled (AWSC) intersections, Chapter Ten Included only capacity guidelines wad no level of service methodology. The recently released 1994 HCM Update remedies some of He deficiencies. A new delay equation for TWSC intersections is introduced and a completely new procedure for capacity and level of service analysis for AWSC ~ntersecdons is added. Deficiencies still remam, however. The TWSC intersection procedures have not been calibrated wad a data base reflecting U.S. driving conditions, He analytical procedure for TWSC intersections cannot account for platooned flow on He major sheet, He methodology fails to provide guidance regarding He use of signalized vs stop-sign control at a given ~ntersecdon, and He data base for AWSC ntersecdons is sdD limited. NCHRP 346 win produce a new version of Chapter Ten of the HCM as well as a new version of He supporting Highway Capacity Software (HCS) Hat will address these deficiencies. The computational procedures included in bow products are based on He first-ever comprehensive data base for traffic flow characteristics at un ign~li~d interaction in He United Sates, including information from 79 TWSC and 41 AWSC intersections. Each site was videotaped for a period of between one and two hours. The final procedures were selected from an evaluation of sixteen candidate models for forecasting capacity and delay at unsignalized intersections. The research complete as part of NCHRP 346 has lead to nine recommendadons from He project Ram, all now approved by the pane} overseeing this project. For TWSC intersections: . . . . . . . The Harders gap acceptance mode! should be used as He basis to compute the capacity of a non-priority traffic stream at a TWSC intersection. New weighing factors for computing the effective conflicting steam flow rate have been identified and documented and should replace those now used in the 1994 HCM Update. New values for the critical gap and follow-up time for each non-priority traffic stream based on geometry conditions and traffic stream composition have been documented and should replace those now used in the 1994 HCM Update. The basic procedure used In the 1994 HCM Update to compute stream capacity should be continued including adjustments to account for impedance and shared lanes. Adjusunen~ to capacity estimates to account for upstream signals should be made if the signal is less than 1200 feet from the intersection. The 1994 HCM Update delay equation should be used to forecast delay. An extension of the current project should be approved for the development of a simulation mode} for TWSC intersections. Existing simulation models do not yield satisfactory results and/or are not comprehensive enough to account for general U.S. conditions.
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4 For AWSC Intersections: The service time model, an extension of Richardson's model, should be used as the basis to forecast departure headways. A set of saturation headways is recommended as input to this model. The Troutbeck equation should be used to forecast delay. This report is the second of two volumes documenting Be work completed for NCHRP 346. Volume one describes the work completed for two-way stop- controlled intersections. Volume two Includes eight chapters documenting Me work completed for all-way stop-controlled intersections. Chapter two describes previous research work or studies that have been completed for AWSC intersections. Chapter three lists He candidate models for capacity and delay Hat were considered. Chapter four describes the field data collection effort that produced the data base used in the mode} testing and validation. Chapter five describes the analysis of saturation headways. Chapter six describes the concepts of service time and capacity and He validation of the service time model. Chapter seven describes the validation of the delay model. Chapter eight documents He computational procedures that will be recommended for inclusion in the HCM.