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39 Use of Microsimulation Methods to, and exit from, the segment, and free-flow speeds of 35 mph or greater. Microsimulation modeling is an analytical process that uses 4. If the use of a simpler technique--even if the inputs are sophisticated computer programs to analyze traffic operations outside of the recommended ranges--yields outputs that for complex roadway systems. In microsimulation modeling, are consistent with observed conditions (e.g., traffic seems individual imaginary vehicles are assigned characteristics, very congested, and use of the HCM methodology yields such as a destination, vehicle performance capabilities, and LOS E or F). driver behavioral profiles. Each "vehicle" then travels through a computerized roadway network, and various aspects of its Guidelines regarding when to consider microsimulation: performance are recorded during its simulated trip based on its interactions with other vehicles and traffic controls. These 1. Signalized or unsignalized intersections that have more performance statistics can be summarized in many ways, than four legs or are oriented in atypical ways. The ana- including performance measures commonly used by traffic lyst should initially attempt to use HCM methodologies engineers and transportation planners (e.g., delays, travel times, and then consider microsimulation modeling if the inputs travel speeds, and queue lengths). required to evaluate the roadway segments do not corre- Some aspects of roadway systems, such as intersections con- spond to the HCM analysis structure. trolled by isolated or coordinated traffic signals, can be ana- 2. Airport roadway segments with the number of lanes chang- lyzed using simpler techniques than microsimulation. The use ing along the length of the segment and with multiple, and of microsimulation models can be beneficial in other roadway possibly unusual, orientations of driveways or intersecting environments, including those with complex traffic move- side roads. ments, such as weaving operations where some vehicles are 3. Weaving segments that do not fit the orientation of the entering, some are exiting, and some are traveling through the weaving analysis in the HCM. This could mean more than weaving sections. two entries or exits, dimensions or speeds outside the Many airport roadway systems are sufficiently complex to bounds defined in the HCM, signals or stop signs within the warrant the use of microsimulation. The use of microsimula- weaving segment, etc. tion models should be considered if simpler analytical tools 4. If simpler techniques are used to analyze what appear to and methodologies do not yield reasonable results, provide be sufficiently simple facilities, but the results indicate oper- sufficient detail, or cannot be used because the roadway con- ations that are much worse or much better than those figuration or operating conditions are outside the range of observed. those addressed in the HCM. However, the use of micro- 5. When congestion on one roadway section causes queues or simulation models and analyses of traffic using these models backups that extend back and interfere with operations on are relatively complex tasks requiring training in the use of an upstream critical roadway section. the specific model and experience in traffic engineering to 6. When congestion on one roadway section significantly fully understand the simulation process so that appropriate restricts the volume of vehicles that can arrive at a down- inputs are used and the outputs are interpreted correctly. Most stream critical location. microsimulation software packages also require significant 7. When comparing the congestion resulting from different time to learn. improvement options for situations where it is not possible Suggested guidelines on when microsimulation is proba- to design sufficient capacity to eliminate significant conges- bly not needed are as follows: tion. In this case, comparisons are typically made of the extent of the congestion (duration and length of queues) 1. Signalized or unsignalized intersections can usually be produced by the various improvement options. Micro- analyzed using methodologies in the 2000 HCM unless simulation modeling is the best available tool for making exclusive left-turn lane storage area overflows are a signif- these comparisons. icant problem. In such cases, HCM methodologies may yield optimistic estimates of signal performance, and micro- FHWA's Traffic Analysis Toolbox III: Guidelines for Applying simulation modeling may yield more accurate results. Traffic Microsimulation Modeling Software (FHWA-HRT-04- 2. Roadway segments having few or no driveways or inter- 040, July 2004) provides additional information on the use and secting side roads. application of microsimulation. This document is available 3. Weaving segments with two entries and two exits and a rea- at http://ops.fhwa.dot.gov/trafficanalysistools/tat_vol3/index. sonable distance (e.g., at least 500 feet) between the entrance htm.