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