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.  I will add to this in another post.

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.

I’ve occasionally seen some road nicknames that are particularly good.  A few that I’ve heard:

• Malfunction Junction (I-275 and I-4, Tampa, FL)
• The Riddle in the Middle (Alaska Way, Seattle, WA)
• Spaghetti Junction (I-85 and I-285, Atlanta, GA)

I’ve also started calling a strech of Columbia Parkway (Cincinnati, OH) “The Suicide Side”, which is a 45 MPH arterial that everyone goes 60 MPH.  The divider is a double-yellow line… only.

Got any more?  Add ‘em in the comments.

Trip generation is likely one of the easiest parts of the four step process.  Normally, the most difficult part of dealing with trip generation is getting the input socioeconomic (population and employment) data correct.  This post explains how trip generation is calculated in the model… Read the rest of this entry »

The center of most travel demand models is the “Four Step Model”.  This model was created in the 1950s to determine the demand on roadways.  The four steps include:

1. Trip Generation
2. Trip Distribution
3. Mode Choice
4. Trip Assignment

Read the rest of this entry »