1. CE 2710 Transportation Engineering
The Four-Step Model
Wesley Marshall, P.E University of Connecticut March 2009

2. The Basic Transportation Model…
Study Area Zones
Attributes of Zones
Socioeconomic Data
Land Use Data
“Cost” of Travel btw. Zones
The Road Network
Inputs
Traffic Volume by Road Link
Mode Splits
Emissions
Outputs

3. What’s in the Black Box?
The Four-Step Model

4. The Four-Step Modeling Process…
Trip Generation
Trip Distribution
Mode Choice
Trip Assignment

5. Overview of the Four-Step Model

6. } } TRIP GENERATION Input:
- No. of Housing Units - Office, Industrial SF
Land Use Data
Household Socioeconomic Data
- HH Size
- Income
- No. of Cars
Examples of HH socioeconomic data }
Examples of land use data
Iterative Process
Output:
Trip Ends by purpose
- i.e. columns of trip productions and trip attractions
TRIP DISTRIBUTION
Input:
Trip Ends by purpose
Output:
Trip Interchanges
- a matrix of trips between each TAZ… also called a “trip table”
MODE CHOICE
Input:
Trip Interchanges
Output:
Trip Table by Mode
- a matrix of trips between each TAZ for each type of mode
TRIP ASSIGNMENT
Input:
Trip Table by Mode
Output:
Daily Link Traffic Volumes
- i.e. traffic flows on network, ridership on transit lines

7. P/A versus O/D Trip Production
Home end of the trip (where the need to make a trip is generated)
Trip Attraction
Non-home end of the trip (where the need to make a trip is satisfied)
Trip Origin
# of trips that begin in a zone
Trip Destination
# of trips the end in a zone

8. Zone 1 Residential Zone 2 Non-Residential
Zone 1 = Production & Origin
Zone 1 = Production & Destination
AM Peak Hour Trip
PM Peak Hour Trip
Zone 2 = Attraction & Destination
Zone 2 Non-Residential
Zone 2 = Attraction & Origin

9. Some General Problems with the Conventional Methodology
Focus on vehicular traffic
Better models typically have a transit component
Typically forecasts huge increases in traffic
Leads to engineers building bigger roads to accommodate “forecast” traffic
Which leads to induced traffic and congestion… right back where we started when we needed the bigger roads in the first place

10. Some General Problems with the Conventional Methodology
Pedestrians and bicyclists are rarely included
Level of geography is difficult for non-motorized modes
Network scale is insignificant
Input variables are too limited

11. Four-Step Model Problem

12. 4-Step Model Problem
Given the zone data above and the tables/charts on the following page, Find the vehicle flows on the road network to the left using “all-or-nothing” traffic assignment.
This will require completing all four steps of the transportation planning process…

13. 4-Step Model Problem

14. Basic Step-By-Step Process
Trip Generation
Using the socioeconomic & land use data along with the cross-classification tables, come up with the number of trip productions & attractions for each zone…
Trip Distribution
Create trip interchange table based upon “friction” between zones using the travel time matrix, the F-Factor graph, and the Gravity Model…
Tij =
Pi Aj Fij
ΣAjFij
(Productions)(Attractions)(Friction Factor)
Sum of the (Attractions x Friction Factors) of the Zones =
Gravity Model
Mode Choice
Create a separate trip interchange table for each mode based upon, in this case, the relationship of land use household density the percentage of people walking & biking…
Trip Assignment
Distribute the vehicle trips to the street network using “all-or-nothing” traffic assignment by assuming that all trips are accommodated on the shortest possible path between zones…

15. 1 2 3 4 Travel Time = 2 Travel Time = 5 Travel Time = 6

16. = 683 1
Travel Time = 2 2
= 986
= 817
Travel Time = 5 95
= 106
Travel Time = 6
= 312
Travel Time = 2
527 32 3 4
Travel Time = 5
131

17. 1 3 4 2