What is Simulation?

Levels of Transportation Analysis

Travel demand models, which are sometimes called static models or deterministic models, assume there is no variability in driver-vehicle characteristics, and they assess the operations of segments in the roadway or transit system rather than modeling the individual vehicles using those roadway system segments. For example, travel demand models calculate and report volume, speed, and volume/capacity ratios for each segment of a freeway or arterial. They are best used to study traffic patterns in a region.

Meso and microsimulation models, on the other hand, simulate the characteristics and interactions of individual vehicles as they travel through the system. They produce trajectories of vehicles as they move through the network, interacting with control devices and with each other. The processing logic for simulation includes algorithms and rules describing how vehicles move and interact, including car following, acceleration, deceleration, lane changing, and passing maneuvers. This analytic method provides more detailed and accurate results for the system being studied as compared to static models. Consequently, the output of properly calibrated simulation models is more accurate, realistic, and detailed than that of deterministic travel models.

Categories of Simulation Tools

Macrosimulation Models (not to be confused with Travel Demand Models)

Macroscopic simulation models are based on the deterministic relationships of the flow, speed, and density of the traffic stream. The simulation in a macroscopic model takes place on a section-by-section basis rather than by tracking individual vehicles. Macroscopic simulation models were originally developed to model traffic in distinct transportation subnetworks, such as freeways, corridors (including freeways and parallel arterials), surface-street grid networks, and rural highways. They are less labor-intensive than meso or micro models. (Macroscopic software tools: FreeVAL, OPT)

Mesosimulation Models

Mesoscopic models combine the properties of both microscopic and macroscopic simulation models. As such, mesoscopic models provide less fidelity than microsimulation tools, but are superior to the typical planning analysis techniques. (Mesoscopic software tools: Aimsun, Dynamit)

Microsimulation Models

Microscopic simulation models simulate the movement of individual vehicles based on car-following and lane-changing theories. These powerful but labor-intensive models are effective in evaluating heavily congested conditions, complex geometric configurations, and system-level impacts of proposed transportation improvements that are beyond the limitations of other tool types. (Microscopic software tools: Aimsun, Vissim, TransModeler)