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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
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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
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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
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Representative terms from entire chapter:
signal timing
