13.4.7 Set up the Simulation and Run the Model

Before running a simulation, set up the desired Simulation Period, Random Seed Increment, and Number of Runs.
  • The Simulation Period describes the total simulation time for each model run in simulation seconds.
  • The Simulation Period is made up of the Seeding Period (time necessary for the network to become filled with vehicles) and Analysis Period (time that the network is evaluated for performance metrics).
  • The Random Seed initializes a random number generator used to vary network conditions assigned using stochastic functions. This creates variations of behavior in the Vissim network that accounts for variations in real-world traffic conditions. The Random Seed Increment defines the difference between different Random Seeds of multiple simulation runs. It is best practice to keep the Random Seed Increment consistent between different models that are compared to one another in the same analysis.
  • The Number of Runs determines the number of Random Seeds for the model to may run if the Random Seed Increment is greater than 0. Multiple model runs help account for variations in network operational performance. The variations between model runs result from variations in driver behavior in different Random Seeds. The Number of Runs are calculated based on the number of initial model runs, the mean and standard deviation of the initial run, t-statistic for the desired confidence level, and the tolerance error. More information is provided in
    Section 13.7.2
    .
Vissim begins each simulation without any vehicles in the model network, so some period of time is necessary to seed the network prior to collecting operational data. It is recommended that this Seeding Period (warmup period) be as long as necessary for the model to reach equilibrium. Equilibrium is reached when the number of vehicles entering the network is approximately equal to vehicles exiting the network. The number of vehicles entering and exiting the network are tracked during simulations to determine an appropriate initialization period. As discussed previously, the model simulation includes the build-up of congestion and the dissipation of congestion. Including this warmup and cool down period helps create a more accurate model.