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Chapter One INTRODUCTION AND OVERVIEW ~ January 1993, the University of Idaho, In cooperation with Kittelson ~ Associates, ~c., Rubr Ur~versi~, and Queensland Untversibr of Technology, initiated work on a project to develop new capacity and level of senice analysis procedures for unsignalized intersections. This project, funded through the National Cooperative Highway Research Program (NCHRP) and designated project 3-46, was one of several research projects identified by the Transportation Research Board's Committee on Highway Capacity and Quality of Service (Committee A3AlO) needed to improve the capacity and level of service analysis techniques available to practicing traffic engineers. It is part of an effort by the Committee to develop a new version of the Highway Capacity Manual (HCM) by the year 2900. While most other chapters of He 1985 HCM were supported by one or more nationally funded research projects, Chapter Ten borrowed heavily from a procedure for two-way stop-controlled (TWSC) intersections originally developed in Germany. Unfortunately, the procedure was backed by only I~m~ted calibration data reflecting conditions found in the United States. For ad- way stop-controDed (AWSC) intersections, Chapter Ten included only capacity guidelines with no level of service methodology. The recently released 1994 HCM Update remedied some of the deficiencies. A new delay equation for TWSC intersections was introduced and a completely new procedure for capacity and level of service analysis for AWSC intersections was added. Deficiencies still remained, however. The TWSC intersection procedures had not been calibrated with a data base reflecting U.S. driving conditions, the analytical procedure for TWSC intersections could not account for platooned flow on the major sweet, the methodology failed to provide guidance regarding the use of signalized vs. stop-sign control at a given intersection, and the data base for AWSC intersections was still vomited. NCHRP 3-46 watt produce a new version of Chapter Ten of the HEM as well as a new version of the supporting Highway Capacity Software (TICS) to address these deficiencies. The computational procedures included in both products were based on the first-ever comprehensive data base for traffic flow characteristics at unsignalized 1: ntersections in the United States, including information from 79 TWSC and41 AWSC Intersections. Each site was videotaped for a period ofbetween 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 completed as part of NCHRP 3-46 lead to nine recommendations from the project team, 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 capacitor of a non- priority traffic stream at a TWSC intersection. New weight factors for computing the effective conflicting stream flow rate have been identified and documented and should replace those now used In the 1994 HCM Update. New values for He critical gap and follow-up time for each nompriority traffic stream based on geometry conditions and traffic steam composition have been documented and should replace those now used in He 1994 HCM Update. The basic procedure used in the 1994 HCM Update to compute stream capacity should be continued including adjus~nents to account for impedance and shared lanes. Adjustments to capacity estimates to account for upstream signals should be made if a signal is less than 1,200 feet (400 m) Tom He intersections. The 1994 HCM Update delay equation Known as Troutbeck's equation) should be used to forecast delay. There is a need for the development of better simulation models for TWSC intersections. Existing simulation models Hat were tested do not yield satisfactory results and/or are not comprehensive enough to account for general U.S. conditions. For AWSC Intersections: · The service time model, an extension of
4 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 1994 HEM Update delay equation should be used to forecast delay. This report is the first of two volumes documenting the work complete for NCHRP 3-46. Volume two descnbes the work completed for all-way stop-controlled intersections. Volume one include ten chapters documenting the work completed for two-way stop- controlled intersections. Chapter two describes the theoretical background of previous research work or studies Hat have been completed for TWSC intersections. Chapter Free lists the candidate models for critical gap and follow-up time estimation as well as for capacity and delay. 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 critical gap and fodow-up times. Chapter six describes He capacity mode! testing process and results. Chapter seven describes delay mode! testing process and results. Chapter eight documents He special conditions while analyzing TWSC intersections. Chapter nine discusses issues of signal warrants and level-of-service concept. Chapter ten provides He recommended computational procedures.
