Step 1: Trip generation - How many trips are generated?
Step one of the process is to determine the number of daily trips that take place in the region. This procedure is called trip generation, and it estimates the number of "trip ends" produced in and/or attracted to each transportation analysis zone (TAZ) in the region. Each trip is made of two "trip ends," one at the production end of the trip and one at the attraction end of the trip.
In the TPB model, eight trip purposes are modeled. Five are for person travel: home-based work (HBW) trips, home-based shop (HBS), home-based other (HBO), non-home-based work (NHW), and non-home-based other (NHO). Two are for truck travel - medium truck and heavy-duty truck - and one is for non-freight commercial vehicles (e.g., delivery trucks, service vehicles). School trips are included in the home-based other trip purpose.
The number of trip ends produced and attracted to each zone is estimated using certain assumptions about the number of trips typically made by each type of household and to each type of destination in the region. We use special factors to account for different rates of trip-making that are characteristic of different parts of the region. These assumptions and special factors are included in the equations used to derive the trips for each zone in the region. For example, a household in an inner suburb such as Arlington County with one car is assumed to make fewer shopping trips than a household in the outer suburbs with two cars. (In general, the level of auto ownership has been found to be a good predictor of household trip rates.)
In the trip generation procedure, one assumes that the land activity (e.g., jobs and households) in each zone results in the "production" and "attraction" of trips (actually trip ends). A trip end that is produced in a zone is called a "production." A trip end that is attracted to a zone is called an "attraction." The convention for whether a trip end is a production or attraction is based on the following two rules: A home-based trip is produced at the home and attracted to the non-home end (regardless of the direction of travel). A non-home-based trip is produced at the origin of the trip and attracted to the destination of the trip.
A set of equations is used to estimate the number of trips produced by and attracted to each zone based on its residential and employment characteristics. These estimates rely on the actual or projected employment in the zone to determine how many workers and shoppers it attracts. The more employment a zone has, the more work trips it attracts. The more retail employees in a zone, the more shopping trips are assumed to be attracted there.
The Version 2.3 Travel Model estimates both motorized person trips and non-motorized person trips. Motorized person trips include both those made in private vehicles (e.g., cars, vans, trucks) and those made in public transit (e.g., buses, subways, commuter rail trains). Non-motorized person trips include walk and bike/bicycle trips. Although non-motorized trips are estimated in the trip generation step, they are not carried further into trip distribution, mode choice, and traffic assignment, due to various challenges (e.g., many of these trips are quite short, often not travelling outside of the zone where they are produced).
When the trip generation procedure is completed, we have an estimate of the number of motorized person trip ends produced and attracted to each zone. The next step, trip distribution, will match up all of the trip ends, creating actual trips.