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SUMMARY OF FINDINGS The 1985 Highway Capacity Manual (HCM) prescribes a methodology in Chapter 9 for estimating the delay and level of service at a signalized intersection as a Unction of the traffic characteristics and the signal timing plan. This methodology is widely used both nationally and internationally and approaches a standard for the evaluation ofthe performance of signalized intersections in the U.S.A. The present treatment of traffic-actuated control in the HCM leaves much room for improvement. The limitations ofthis treatment will be the main target ofthe research described in this report. These limitations are largely the result of an oversimplified approach to analysis that has not kept pace with the rest of HCM Chapter 9. The procedures for estimating both the timing plan and delay have been widely criticized for their simplistic nature and their lack of agreement with real world data. The work that was carTied out under NC=P Project 3-48 included the fo~ulation of an ~rnproved methodology for predicting the operating characteristics oftraffic-actuated control, and for estimating the delay and level of service associated with a given set of operating parameters. The principal product of this research was an improved mode! set in a computational framework that facilitated testing of the model, and will eventually become a too! for the end user. The testing and evaluation of the methodology relied heavily on simulation, augmented by limited field studies. The tests indicated that the proposed mode! was robust and produced data that agreed much more closely with the simulation results. The mode! demonstrated its ability to perform properly under a wide variety of conditions, including two phase operation, protected left turns, lead lag phasing, protected plus permitted phasing and coordinated operation. Based on the findings presented in this report, it is suggested that the traffic-actuated control analysis methodology developed in connection with this project is valid and satisfies the project objectives. The signal timing estimation mode! has demonstrated a clear superiority over the status quo. Of particular importance are the mode} features that make it sensitive to the traffic-actuated controller settings, the tree queue phenomenon and the coordinated control mode. The combination of these features gives the mode} capabilities that are needed by the analyst and cannot be found together in any other technique. All analytical traffic signal delay models are formulated as the sum of two components. The first deals with the assumption of completely uniform arrivals in the traffic stream, and the second adds a correction factor to account for the randomness that actually occurs. The analysis procedures developed under this project produced an improved estimate of the uniform delay by modeling the rules of operation of a traffic-actuated controller. Two models for evaluating the random delay component were evaluated and compared using simulation. The first was developed in Australia, and the second was developed in the U.S.A. These models produced very similar results arid no performance basis could be found for recommending one over the other. In view ofthis, the simpler of the two models was recommended for adoption in the NCHRP Project 3-48 Final Report: Page 1
next update of the HCM, with further consideration given to the advantages and disadvantages of these models for the HCM 2000. The role of simulation as a surrogate for field data collection was demonstrated very clearly by this project. What is less clear at this point is the role of simulation as an alternative to the analytical modeling process. There is no question that simulation models such as NETSIM are able to address a much broader range of problems than analytical models. The main question at this point is how to resolve the differences that will inevitably occur in a comparison of the results obtained from these two techniques. Four specific sources of disparity between the NETSIM and HCM delay estimation results were identified. · Differences in the definition of delay; · Differences in the treatment of lost time; · Assignment of delay near the stop line of one signal to the next signal by NET SIM; and · Differences in the distribution of arrivals as viewed by the two methods. A new mode! called EV1PAS was also investigated to determine similarities and differences with respect to the analytical mode! developed as a part ofthis project. EV1PAS offers a self-contained alternative to the design and evaluation of a timing plan for a traffic-actuated intersection. Compared to NET SIM' it offers the advantage of a timing plan design, in addition to the evaluation. This study indicated a high level of agreement in the timing plans produced by NET SIM and EVIPAS. Studies reported in the literature indicate that EV1PAS produces timing parameter settings that are generally superior to venous analytical methods. The traffic actuated timing estimation procedure described in this report provides a reasonable approximation of the operation of a traffic-actuated controller for nearly all of the conditions encountered In practice. The results obtained from this method have correlated well with extensive simulation data and with limited field studies. However, the procedure involves a deterministic analytical representation of an extremely complex stochastic process, and therefore has some Irritations which must be recognized. These include: · Inability to mode! protected plus permitted leg turns from shared lanes. This is a also a limitation of the HCM Chapter 9 mode} itself · Lower limit on the range of unit extension times that can be modeled realistically. · The current HCM mode] for the treatment of shared lane permitted left turns does not lend itself well to this iterative procedure, and discrepancies with the HCM results sometimes occur. The result is that the timing plan produced by the mode! is not optimal with respect to the HCM Chapter 9 analysis of delay. NCHRP Project 3-48 Final Report: Page 2
When traffic volumes are extremely low (i. e., level of service A), the cycle length and green times are not meaningful because of long dwell times in which a red signal is displayed to approaches with no traffic. The principal product of this project is software dependent. The iterative nature of the analysis procedure does not lend itselfto manual implementation. It has therefore been developed as a stand- alone application that will be very useful in practice regardless of any decisions concerning its adoption as an official part of the HCM Chapter 9 procedure. In its present form, it could be distributed and used electively by traffic engineers immediately. Its utility to the profession would be greatly enhanced, however, if it were adopted by the Capacity Committee. NCHRP Project 3-48 Final Report: Page 3