Travel Demand Modeling 101 Part 1: Terminology

It occurred to me that many people likely do not understand all of the terminology of travel demand models.  Because of this, I felt the need to list many of them here. Assignment: A process that determines the specific route of travel from one zone to another.

Binomial Logit Mode Choice: A process that compares the utility of two travel mode choices and determines the probability of using each mode.

BPR Equation: An equation used to determine the speed on a network link based on the ratio of the volume to the capacity.

Centroid: A point that is linked to a TAZ that loads traffic onto the network

Centroid Connector: The link that connects a centroid to the rest of the network.

Equilibrium Assignment: An assignment method that iteratively assigns and reassigns traffic based on revised travel times on each link.

Friction Factors: Factors that are used in distribution to refine the distance measure between zones.

The Four Step Process: A process of modeling that began in the 1950s that performs four major steps: trip generation, trip distribution, mode choice, and assignment.

Fratar Model: An iterative gravity model used to determine trip interchanges where total productions and attractions are known, but not necessarily balanced.

Gravity Model Distribution: A process to determine the likelihood of traveling from one zone to another based on the attractiveness of the destination compared to the distance from the origin to the destination.

Link: A piece of roadway in the network

Mode Choice: A process that determines the economic utility of all available forms of travel from one zone to another and splits trips (from the trip distribution step) into a variety of modes (depending on those that exist).

Multinomial Logit Mode Choice: A process that compares the utility of multiple travel mode choices where each is a separate and distinct choice.

Nested Logit Mode Choice: A process that compares the utility of multiple travel mode choices as a string of choices and sub-choices.  An example would be a structure illustrated below (click on the image for a larger view).

Network: A collection of links, nodes, centroids, etc. that simulate the major travel facilities in the model area.

Traffic Analysis Zone (AKA: zone, TAZ): A geographic division, much like a census block, that defines an area of homes and businesses used in the travel demand model.

Trip Generation: A process of counting the homes, employees (usually by type), schools, and other activities that generate traffic to and from them.  Trips are generated as attractions and productions.

Trip Distribution: A process of determine where generated trip productions are linked to generated trip attractions.  This determines the number of people that will go from one zone to another.