30-35 Washburn Feature

ITE JOURNAL / MARCH 2002 INTRODUCTION The determination of delay at signalized intersections is a relatively complicated task. With each subsequent release of the Highway Capacity Manual (HCM),1 the procedures generally get more accurate but also more complex. Consequently, transportation engineers typically rely on software implementations of the HCM procedures to perform signalized intersection analyses. The use of software packages has also enabled transportation engineers to perform much more complex analyses, such as signal coordination, with relative ease. Although computer software usually reduces the time and effort to perform an analysis, it also distances the user from the underlying analysis principles or methodologies that determine the results. The accepted standard for isolated signalized intersection analysis is the 1997 HCM, specifically Chapter 9. Many software implementations of this chapter have been produced, but the most commonly used is the Highway Capacity Software (HCS).2 Since Chapter 9 of the HCM is really geared towards individual intersection analysis and does not provide for explicit modeling of coordinated intersections, other software packages have been developed for this purpose. For years, TRANSYT-7F3 has been the industry standard for performing this task. Due to the general complexity of input file coding for this program, some other programs with more “user friendly” input editors have started to increase in popularity. At this point, one of the most popular alternatives to TRANSYT-7F is the program named Synchro.4 Although software packages such as TRANSYT-7F and Synchro are the preferred packages by transportation engineers for analyzing a coordinated series of signals, many public agencies stipulate the HCM/HCS as the standard for reporting intersection delays and the corresponding levels of service. Therefore, unless a software program designed for modeling a system of signals also reports delay in a manner consistent with the HCM, the user must either also perform individual intersection analyses in HCS or estimate arrival types for the coordinated conditions and perform just the isolated intersection analyses in HCS to conform to the public-agency requirements. To many transportation engineers, this would be perceived as a duplication of effort. Therefore, most, if not all, users would prefer to use just a single software package like TRANSYT-7F or Synchro for coordinated signal arterial analysis and use the individual intersection delay estimates from these packages for reporting to public agencies as part of traffic-impact studies. Thus, a common question among the practicing profession pertains to how closely the signal delay estimates given by TRANSYT-7F and Synchro compare with those given by the HCS. This feature has two purposes: (1) to provide the reader some background on the analysis methodologies used in the three programs discussed in this feature and (2) to compare the primary measure of signalized intersection performance, control delay, reported by the three software packages. The comparisons in this feature were facilitated by application of the programs to the same set of conditions—a coordinated signalized arterial in the University District of Seattle, WA, USA, as well as previous author experience with the programs. This feature also discusses some of the important program features/factors to consider when choosing which program to use. Signalized Intersection Delay Estimation: Case Study Comparison of TRANSYT-7F, Synchro and HCS