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Wesley Marshall, P.E. University of ConnecticutMarch 2009
CE 2710Transportation Engineering
The Four-Step Model
The Basic Transportation Model…1) Study Area Zones 2) Attributes of Zones
Socioeconomic Data Land Use Data “Cost” of Travel btw. Zones
3) The Road Network
Traffic Volume by Road Link Mode Splits Emissions
Inpu
tsO
utpu
ts
What’s in the Black Box?
The Four-Step Model
The Four-Step Modeling Process…
I. Trip Generation
II. Trip Distribution
III. Mode Choice
IV. Trip Assignment
Overview of the Four-Step Model
- i.e. traffic flows on network, ridership on transit lines
- a matrix of trips between each TAZ… also called a “trip table”
- i.e. columns of trip productions and trip attractions
- No. of Housing Units- Office, Industrial SF
- HH Size- Income- No. of Cars
Iter
ativ
e P
roce
ss
Land Use Data
Input: Household Socioeconomic Data }Examples of HH socioeconomic data
}Examples of land use data
Output: Trip Ends by purpose
Input: Trip Ends by purpose
Output: Trip Interchanges
Input: Trip Interchanges
Output: Trip Table by Mode
Input: Trip Table by Mode
Output: Daily Link Traffic Volumes
TRIP GENERATION
TRIP DISTRIBUTION
MODE CHOICE
TRIP ASSIGNMENT
- a matrix of trips between each TAZ for each type of mode
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
Zone 1Residential
Zone 2Non-Residential
Zone 1 = Production & Origin
Zone 2 = Attraction & Destination
Zone 2 = Attraction & Origin
Zone 1 = Production &
Destination
AM Peak Hour Trip
PM Peak Hour Trip
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
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
Four-Step ModelProblem
Zone Information
Zone 1 2 3 4
Households 250 500 300 100
Area (sq. miles) 1.0 0.5 1.0 2.0
Retail Employees 65 390 215 100
Non-Retail Employees 400 300 85 10
% Low Income 40% 20% 50% 20%% Medium Income 60% 30% 50% 80%
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…
Travel Time = 21
3 4
2
Tra
vel T
ime
= 6
Tra
vel T
ime
= 5
Travel Time = 5
Travel
Time =
2
F Factor vs. Travel Time
0
10
20
30
40
50
60
70
80
90
0 2 4 6 8 10
Travel Time (min)
F F
acto
r4-Step Model Problem
% of HH in Each Income Category Versus Auto OwnershipAutos/HH
0 1 2+Income
Low 50 40 10Medium 10 50 40
High 10 30 60
# of Trips per Household per DayAutos/HH
0 1 2+Income
Low 2 4 8Medium 4 5 10
High 4 5 8
Trip Attraction Rates
Attraction perper HH 1.0
Retail Employee 5.0Non-Retail Employee 2.0
Travel Time Between Zones
Zone 1 2 3 4
1 2 2 4 8
2 2 4 2 6
3 4 2 1 5
4 8 6 5 2
Alternative Modes vs. Density of Attraction Zone
0
5
10
15
20
25
30
35
40
45
0 200 400 600 800 1000
Household Density (HH / sq. mi.) of Attraction Zone
% W
alki
ng
& B
ikin
g
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…
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…
Gra
vity
Mod
el
Tij = Pi Aj Fij
ΣAjFij
(Productions)(Attractions)(Friction Factor)Sum of the (Attractions x Friction Factors) of the Zones=
Travel Time = 21
3 4
2
Tra
vel T
ime
= 6
Tra
vel T
ime
= 5
Travel Time = 5
Travel
Time =
2
Travel Time = 2
787 + 159 + 40 = 986 1
3 4
2
Tra
vel T
ime
= 6
Tra
vel T
ime
= 5
Travel Time = 5
Travel
Time =
2
586 + 97 = 683
131
32
950
97 +
215
= 3
12
40 +
66
= 1
06
527159 + 658 =
817
1
3 4
2