? Provides a cost effective wayof testing and evaluating different scenarios ? Allows the user to testscenarios faster than in real life ? Offers an insight into thecharacteristics of traffic system operations that are important, allowing theuser to make a more informed decision ? Provides outputs/animationthat the public can understand At the same time simulation has somedisadvantages: ? Requires collection ofdetailed data on field conditions ? Needs calibration andvalidation of the model prior to testing scenarios ?Requires an understanding of how the model works before assessing the outputsThe processes of conducting a simulation study are described in the followingsection. : The movement of vehicles in a microscopic simulation program iscontrolled by different logic. There might be logics for car-following, lanechanging, merging, behavior on freeways and urban routes. It is necessary tohave a basic understanding of the logic being applied in the simulation modelbefore selecting it. It is also helpful to know if the user is given control inmodifying or changing the logic in the model. Ease of use: Each simulationmodel is different in terms of its ease of use.
Some simulation models providean easy to use graphic user interface (GUI) and have a simple process for datainput. Some models allow the user to use an aerial photograph or map as abackground. while coding the network, which makes geometry coding much easier.And some models are easy to calibrate/validate by providing different measuresof effectiveness in a readable format. Simulation models provide users withanimation of the network. Some models have the capability to provide 3-dimensional animations with other details (like buildings, traffic signals,etc.
) and also to allow the user to zoom or pan across networks. Most of theseanimations can be made into movie clips and are great tools for presentations,providing a vivid presentation of the benefits of TSP to policy boards andpublic. Most simulation models have a provision that allows for customizingoutput format. It helps the user to have the results in a compiled form ratherthan a vehicle-by-vehicle or a link-by-link basis. Sometimes the models alsosupport tracking individual vehicles and this might be useful in the case oftransit signal priority.
Travel time, delay, queue length, number of stops,fuel consumptions, emissions, bus/tram wait time are some of the most usedoutputs. Field data is required to conduct a simulation study. The data usuallyrequired to conduct a simulation is as follows: ? Geometric — length of lanes,lane widths, number of lanes, lane assignment, length and location ofcrosswalks ? Signal timing plans – cycle lengths, number ofphases, duration of phases, etc. for different times of the day ?Speed – speed limits and average vehicular speed ? Transit vehicle – traveltimes, dwell times and distribution of dwell times The actual amount of datacollected depends on available budget, time and project requirements. At aminimum a.m. and p.
m. peak data should be collected. If there is a significantvariation in traffic volumes, additional data may need to be collected such asmidday, Saturday and Sunday.
Attention should be given to the time period ofdata collection. For example, data collection of regular weekday traffic shouldbe conducted between Tuesday and Thursday to avoid traffic fluctuations due toweekend trips and preferably, should not be during a holiday season. Weekenddata should be collected during times that are most relevant to meet projectobjectives, such as weekend traffic peak period and/or weekend peak transitoperational headways Developing the network for simulation involvesthe processes of coding, calibration and validation.
Calibration and validationare extremely important, but often do not get the attention they deserve in thesimulation process. Coding: Coding is the process of building the network inthe simulation model. It involves input of the study area geometry in thesimulation model with other information like the timing plans and volumes.
Given below are the different ways to code a network in a simulation model: ?The network can be coded in the simulation model itself. ?The network can be coded in a Network Editor that is bundled with thesimulation model and can then be transferred to the simulation model. TheNetwork Editor is designed to simplify network coding. ? Insome cases the network can be coded in simulation or optimization models andcan be transferred/exported to other simulation models.
This procedure ismainly used when multiple softwares are required for the analysis andoptimization of the network. Sometimes, due to the simplicity of the NetworkEditor, the study network is coded in one model and then transferred to anothermodel. If this method of coding the network is used, the user should be awareof the features/elements that may be altered or missed during the transferprocess.
Calibration: Simulation models need to be calibrated before they areused for testing future scenarios since an un-calibrated model might lead toskewed results. Calibration involves changing some of the default parametersused in the simulation model so that it reflects the conditions observed in thestudy area. As previously mentioned, the movement of traffic in a simulationmodel is governed by different logics.
Simulation models provide the user withdifferent parameters to fine tune the logic and other aspects of the model.Examples of these parameters include minimum headway, minimum accelerationrate, minimum deceleration rate, maximum acceleration rate, etc. These parametersare give