Transportation forecasting uses a four step model to estimate future travel demand: 1) Trip generation estimates the number of trips originating and ending in each traffic analysis zone based on socioeconomic and land use data. 2) Trip distribution determines trip destinations from origins. 3) Mode choice identifies the transportation mode for each trip. 4) Route assignment specifies the routes for each trip. Trip generation models predict trip "ends" as either origins and destinations, with origins at residential zones and destinations at non-residential zones, or as productions at residential zones and attractions at non-residential zones.

1. Transportation Forecasting
What is it?
Transportation Forecasting is used to estimate the
number of travelers or vehicles that will use a given
transportation facility in the future
The conventional approach to transportation
forecasting is based on what is commonly know as
the ‘four step model’

2. The Four Step Model
• Trip Generation
Estimates the number of trips from given origins and
destinations
• Trip Distribution
Determines the destination for each trip from a given origin
• Mode Choice
Determines the mode choice for each trip
• Route Assignment
Determines the specific route for each trip

3. Trip Generation
Trip Generation model is used to estimate the number of person-trips that
will begin or end in a given traffic analysis zone
The unit of analysis for traffic generation is the TAZ
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4. Trip Generation Model
Calibrated
Model
Estimated
Target year
socio-economic,
land use data
Predicted
Target year
No. of Trips

5. Trip Generation
What is Predicted?
Trip generation models predict so called TRIP ENDS for each zone
The trip ends maybe classified as either
• ORIGINS and DESTINATIONS (O-D)
or
• PRODUCTIONS and ATTRACTIONS
The two sets of terms sound similar but there is a technical difference

6. Origins and Destinations
A worker leaves Zone 1 in the morning to
go to work in Zone 8
This results in 2 trip ends:
• One Origin for Zone 1
• One Destination for Zone 8
1
8
Residential
Non-residential
Residential
Non-residential
When that same worker leaves Zone 8 in
the evening to go to home to Zone 1
This results in another 2 trip ends:
• One Destination for Zone 1
• One Origin for Zone 8
Total Number of Trip Ends
Zone 1: 2 Trip Ends (1 O, 1 D)
Zone 8: 2 Trip Ends (1 O, 1 D)

7. Productions and Attractions
A worker leaves Zone 1 in the morning to
go to work in Zone 8
This results in 2 trip ends:
• One Production for Zone 1
• One Attraction for Zone 8
1
8
Residential
Non-residential
Residential
Non-residential
When that same worker leaves Zone 8 in
the evening to go to home to Zone 1
This results in another 2 trip ends:
• One Production for Zone 1
• One Attraction for Zone 8
Total Number of Trip Ends
Zone 1: 2 Trip Ends (2 Productions)
Zone 8: 2 Trip Ends (2 Attractions)

8. Origins and Destinations??
Productions and Attractions??
Based on the convention of trip generation models
Origins and Destinations are defined in terms of the direction of
the trip
Productions and Attractions are defined by the land use
Residential Land use PRODUCES trip ends
Non-residential land use ATTRACTS trip ends
This is a useful distinction because of how trip generation models
are typically developed based on this.