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What is Traffic Modeling?

The Highway Capacity Manual (HCM) describes a simulation model as "A computer program that uses mathematical models to conduct experiments with traffic events on a transportation facility or system over extended periods of time." Simulation models are designed to emulate the behavior of traffic in a transportation system over time and space to predict system performance. A simulation model studies a real-world system, taking into consideration the mathematical and logical concepts associated with the operation of the system, and emulates it with intricate computer software. Simulation model runs can be viewed as experiments performed in the laboratory rather than in the field. Simulation models are typically classified according to the level of detail at which they represent the traffic stream.

Microsimulation Models

Definition: Microsimulation is the dynamic and stochastic (random – see note below) modeling of individual vehicle movements within a system of transportation facilities. Each vehicle is moved through the network of transportation facilities on a split second by split second basis according to the physical characteristics of the vehicle (length, maximum acceleration rate, etc.), the fundamental rules of motion (e.g. acceleration times time equals velocity, velocity times time equals distance) and rules of driver behavior (car following rules, lane changing rules, etc.).

This definition was taken from the California Department of Transportation publication, “Guidelines for Applying Traffic Microsimulation Modeling Software” which was published in partnership with Dowling Associates in September 2002.

Note: Stochastic is synonymous with random. The word is of Greek origin, means "pertaining to chance", and is often used in mathematical and scientific applications.

Macroscopic Models

Macroscopic models simulate traffic flow, taking into consideration cumulative traffic stream characteristics (speed, flow, and density) and their relationships to each other. The simulation in a macroscopic model takes place on a section-by-section basis rather than by tracking individual vehicles. Macroscopic models employ equations on the conservation of flow and on how traffic disturbances broadcast through the system like shockwaves. Macroscopic simulation models were originally developed to model traffic on distinct transportation sub networks, such as freeways, corridors (including freeways and parallel arterials), surface-street grid networks, and rural highways. They can be used to predict the spatial and sequential extent of congestion caused by traffic demand or incidents in a network; however, they cannot model the interactions of vehicles on alternative design configurations.

Mesoscopic Models

Mesoscopic models combine the properties of both microscopic and macroscopic simulation models. Mesoscopic models are somewhat less consistent than microsimulation tools, but are superior to some other traffic analysis techniques. These models simulate individual vehicles, but describe their activities and interactions based on aggregate (macroscopic) relationships. They can simulate the routing of individual vehicles equipped with in-vehicle, real-time travel information systems. The travel times are determined from the simulated average speeds on the network links, which, in turn, are calculated from a speed-flow relationship. Typical applications of mesoscopic models are evaluations of traveler information systems.