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60 10 Operations Considerations 10.1 Operational Impacts Associated with Signal Phase Mode and Sequence The selection of phase mode and sequence at a traffic signal has a significant impact on operational performance. As mentioned in detail in Chapters 4 through 7 of this Guide, phase mode and sequence can either increase or degrade traffic operational efficiency depending on the characteristics of an intersection. Signal timing parameters, including intersection timing objectives such as isolation, corridor coordination, corridor progression, and equitable side-street access, are critically intertwined with signal phasing mode and sequence decisions. While the individual impacts of a phasing, sequence, or timing decision might be understood or estimated in isolation of the others, the process of determining an intersectionâs phasing must consider the operational interaction between left-turn phasing, right-turn phasing, pedestrian phasing, phasing sequence, and signal timing. Three methods of determining this interaction are provided below. 10.2 Methods of Operational Analysis 10.2.1 Deterministic The most common method of evaluating the operational performance of different phase modes, sequences, and timings is traffic analysis of the specific intersection(s). This analysis uses a deterministic methodology most often applied by traffic software such as Synchro, VISTRO, or HCM Streets. The deterministic methodology applies analysis techniques set forth in the HCM. It is limited in application because it applies uniform driver-vehicle characteristics and is only appropriate to use in undersaturated traffic conditions, among other reasons. Engineers use these traffic software programs to obtain operational metrics such as vehicle delay, queue lengths, volume-to-capacity ratio, and phase failure. A comparison of phasing, sequence, and timing strategies can be made from these metrics. Each strategy should also follow several objectives, including incorporating requisite pedestrian crossings, accommodating queue lengths within available queue storage, mitigating the potential for phase failure, and maintaining intersection and corridor signal timing goals. Deterministic analysis methods require a fair amount of data collection, notably current traffic volumes, lane configurations, and existing signal phasing, sequence, and timing parameters. 10.2.2 Microsimulation In certain situations, deterministic traffic software tools are not suitable for signal operations analysis. These situations include oversaturated traffic conditions, unique intersection design (e.g., various types of alternative intersections), and variable driver-vehicle characteristic behavior. In these situations, stochastic microsimulation traffic modeling software such as SimTraffic, VISSIM, or CORSIM can be used to model the operations of a signal, corridor, or network. The output metrics and objectives of a microsimulation analysis are similar to those of a deterministic analysis; the few differences may include ability to mitigate phase failure (due to oversaturated traffic conditions) and the ability to capture
61 additional metrics such as vehicle stops and travel time. A critical difference between deterministic and microsimulation analysis is the increased amount of data, labor, technical skill, and thus cost required to produce an accurate calibrated microsimulation model.
