Step 2: Trip distribution - Where do the trips go?
In step two, trip distribution, the motorized person "trip ends" developed in trip generation are linked geographically into complete trips, from an origin/production zone to a destination/attraction zone. For example, the work trip ends "produced" by a zone in Gaithersburg, Maryland are matched with the work trip ends "attracted" by other zones throughout the region. When trip ends are linked to create complete trips, the linking may occur within the same zone, or between adjacent zones, or between zones that are some distance apart, such as a zone in Gaithersburg and a zone in downtown Washington, D.C. The same process is used to connect all of the trip ends produced in or attracted to various zones in the region into complete trips.
The modeling process for trip distribution relies on the general assumption that time spent traveling is perceived negatively; the more distant the destination, the more burdensome the trip. Most of the trips produced in a given zone will be attracted to surrounding or nearby zones; some will be attracted to moderately distant zones; and very few will be attracted to very distant zones. (The effect of travel time in discouraging trips is more pronounced for non-work trips than for commute trips, where trips fall off less sharply with distance.)
This general principle is translated into a mathematical procedure known as a "gravity model," with the effect of travel time represented as a "friction factor." Once completed, this procedure yields a set of motorized person trips, contained in "trip tables," made from and to each zone in the region. Trip tables are produced for each trip purpose.
In the next step, the mode choice model is used to predict the likely travel modes of all travelers in the region.