Trip distribution is a model that distributes trips between origin and destination zones based on trip production and attraction models. It balances productions and attractions within each zone by ensuring they are equal and proportionately adjusting attractions to match productions. The most widely used trip distribution method is the gravity model, which distributes trips throughout the transportation network based on travel time and other socioeconomic factors between zones.

1. Trip Distribution

2. Balancing Productions and Attractions
• Prior to proceeding to the trip distrubution step, it is
important to ensure that the productions and attractions
within the study are equal.
• Productions are generaly considered to be more accurate.
• Attractions are balanced proportionately to match
productions within each zone.

3. Balancing Home-Based Work Trips

4. Definition of Trip Distribution
• Trip distribution is a model of the number of trips that occur
between each origin zone and each destination zone. It uses the
predicted number of trips originating in each origin zone (trip
production model) and the predicted number of trips ending in
each destination zone (trip attraction model).
• It is the process by which all trips generated in a study area are
allocated among zones.
• The most widely used T/D method is the Gravity Model, which
uses the attributes of transportation system and land use
characteristcs to distribute trips throughout the network.

5. Gravity Model

6. Gravity Model
• Friction Factor
– It is determined by
calibration process
• Inversely related to travel
time

7. Socioeconomic Adjustment Factor
• Used to make adjustments
of trip distribution values
between zones where:
– differences between
estimated and actual values
are significant
– Accounts for variables other
than travel time

8. Gravity Model - Example problem

9. Gravity Model - Example problem

10. Gravity Model - Example problem

11. Gravity Model - Example problem