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The Four-Step Travel Model

GEOG 111 & 211A – Fall 2006

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Outline

• Background• Role of Simulation• General Process• Example of Most Popular Simulation Model• Examples of Other Ideas• Summary

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Needs• Identify Projects for the Region• Use Formal & Accepted Technique(s) to

Estimate Project Impacts• Simulate the Region for the Next 20

Years• Create Scenarios for BEFORE and

AFTER a Project or Group of Projects

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• Create a Comprehensive Plan of how we want an area/region to be in the future

• Propose projects designed to achieve the goals of the Comprehensive Plan

• Test Scenarios implementing different projects and forecast their effects

• Determine what projects should be continued to next stage

• Present recommendations to decision makers

How Do we use Simulation Models?

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Project Types• New Highways (e.g., bypass-ring roads)• New Management Activities (e.g., park &

ride, signal systems )• New Land Uses (e.g., a new industry, a

new residential neighborhood)• New Technologies??????? (maybe in

management)

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The Context• Urban Transportation Planning System (UTPS) &

Urban Transportation Modeling System• TEA 21 made it also A Statewide Transportation

Planning System• Technology should be added (see Pennplan) • Associate quantitative estimates with performance

measures as in the monitoring part (see PennPlan)• Four-step scheme followed today in many MPOs• Four-step travel model is limited but popular!

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Goals andobjectives

SurveillanceReappraisalProcedural developmentServiceAnnual report

Continuing elements

Policy andtechnical development

Develop immediateaction plan

PennDOTMPOsTMAsLDDsLocal gov’tsCitizen participation

Transportationorganizations

Plan implementation

Land useTrip generationTrip distributionModal splitTraffic assign.

Calibrate models

PopulationLand useEconomicTrafficRevenues

Areawideforecast

= Part of the Sequential Demand Forecasting Process

Develop alternativesApply models

Land useTrip generationTrip distributionMode choiceTraffic assign.

Plan testing, evaluationand selection

Analysis of futurealternative systems

PopulationLand useEconomic activityTransportation systemTravel volumesTerminal and transfer

facilities status & useFinancial resourcesCommunity values

Inventories anddata collection

DATA

Typical Process in Long Range Planning

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The Sequential Forecasting Process & the Urban Transportation Planning System (UTPS)

[adapted from Papacostas & Prevedouros, 1993]

Transportationsystem

specification

NetworkAssignment

ModeChoice

TripGeneration

TripDistribution

Land use and socioeconomic

projections

Direct (user)Impacts

4-STEP

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Four step in large MPOs• Inventory of facilities• Opportunity to think strategically• Show the impact of projects on air quality• Provide report of emissions inventory• Tool for policy assessment• PSRC example follows!

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• Forecast future development (business, roadways, and housing)

• Model the area’s traffic network• Estimate model of the area’s traffic network in the future• Make changes to network characteristics in the future

model• Compare network performance under different scenarios of

project development

What Other Steps Are Required?

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UTPS Outline

• Review the Data Inventory for a Region• Review one Procedure to Predict Future Volumes on

a Highway• Summarize the Method Known as UTPS• Questions

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UTPS 4-step Travel Model

• Trip Generation• Trip Distribution• Modal Split• Traffic Assignment

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Data (Inventory)• Area of study definition & traffic analysis zones• Area of study description (highways, facilities, zoning,

rules/regulations)• Who are the residents? (age, gender, education,

employment, income of residents)• Where do people leave?• Where do people work, shop, etc?• Highway characteristics• Other information (plans for rezoning)

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Travel Model

• Urban Travel Model (‘60s)• Also known as the Four - Step Process• A methodology to model traffic on a network

• General planning & programming• Land use forecasting (where will residences and stores

be?)• Four Steps:

• Trip Generation Estimate Person Trips for each TAZ• Trip Distribution Distribute Person Trips from TAZ to TAZ• Mode Choice Convert Person Trips to Vehicle Trips• Traffic Assignment Assign Vehicles to the Network

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Key Concepts of 4-step

• TAZ: Traffic Analysis Zone• TAZ= a common sense subdivision of the study area• TAZs are used in trip generation and trip distribution• TAZs may be any shape or size, but US Census Blocks, Block Groups,

and Tracts are often used (class: WHY?)

Block Block Group Tract

i.e., a city block

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8-Zone Study Area with Traffic Analysis Zones (TAZs)

8-Zone Study Area

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87

4 5

6

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z

z = all zones outside study areax = TAZ designations study area boundary

TAZ boundaries

Legend:

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Key Concepts of UTPS

• Centroid• Every TAZ (Gate and Internal) has a centroid, usually placed

roughly at the geographic center of the TAZ• All trips to or from a TAZ are assumed to start or end at the

centroid

• Discussion• Why do we use TAZs and centroids to model trips?

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Key Concepts of UTPS• Gate TAZs

• TAZs placed outside the Study Area where major roads cross the boundaries of the study area

• Used to model External Trips (i.e., trips with an origin or destination) Note: Do we care when OD is outside the study area?

• Gate TAZs represent all areas outside of the study area

(Study Area)

Gate TAZ

Network

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Network & Links Numbered

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3 45 6 7

8 9 10 11 1213 14 15

16 1718 19

20 21

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Example: Computer schematic representation

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1

2

3 4 5 6 7

8 9 10 11 12 13

14 15 16 17 18

19 20 21

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23Network and Nodes Numbered

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Gate TAZ

Centroid

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Land Use/Economic Analysis• Considers general trends• Allocates population to geographic

subdivisions• Assigns land uses• Each TAZ contains “observed”

numbers for today’s analysis and predicted numbers for forecasting

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OUTPUT = a map with TAZ’s and their characteristics –

households, businesses, employers

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Trip Generation Process

• Collect Data, usually by Surveys and Census• Sociodemographic Data and Travel Behavior Data

• Create a Trip Generation Model (e.g., regression)• Estimate the number of Productions and Attractions

for each TAZ, by Trip Purpose• Balance Productions and Attractions for each Trip

Purpose• Total number of Productions and Attractions must be equal

for each Trip Purpose• We will discuss balancing later

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Trip Generation Models

• Regression Models• Explanatory Variables are used to predict trip generation

rates, usually by Multiple Regression• Trip Rate Analysis

• Average trip generation rates are associated with different trip generators or land uses

• Cross - Classification / Category Analysis• Average trip generation rates are associated with different

trip generators or land uses as a function of generator or land use attributes

• Models may be TAZ, Household, or Person - Based

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ITE Trip Generation Manual

• Trip Rate Analysis Model• Univariate regression for trip generation• Primarily for Businesses• Explanatory variables are usually number of employees or

square footage• Models developed using data from national averages and

numerous studies from around the US

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Possible geographic detail in trip generation with GIS & business data

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Typical output from trip generation

OUTPUT

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Trip Distribution• Convert Production and Attraction Tables into

Origin - Destination (O - D) Matrices

Destinations

123456Sum

Origins 1 2 3 4 5 6 SumT11 T12 T13 T14 T15 T16 O1

T21 T22 T23 T24 T25 T26 O2

T31 T32 T33 T34 T35 T36 O3

T41 T42 T43 T44 T45 T46 O4

T51 T52 T53 T54 T55 T56 O5

T61 T62 T63 T64 T65 T66 O6

D1 D2 D3 D4 D5 D6

TAZ P1513261881393

A2265522693

123456Sum

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Trip Distribution, Methodology

• General Equation:• Tij = Ti P(Tj)

• Tij = calculated trips from zone i to zone j• Ti = total trips originating at zone i• P(Tj) = probability measure that trips will be attracted to

zone j

• Constraints:• Singly Constrained

• Sumi Tij = Dj OR Sumj Tij = Oi• Doubly Constrained

• Sumi Tij = Dj AND Sumj Tij = Oi

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Trip Distribution Models Example

• Gravity Model Tij is:• Tij = trips from zone i to zone j =

• Ti = total trips originating at zone i• Aj = attraction factor at j• Ax = attraction factor at any zone x• Cij = travel friction from i to j expressed as a generalized cost

function• Cix = travel friction from i to any zone x expressed as a

generalized cost function• a = friction exponent or restraining influence

Sum (Ax / Cix)a TiAj / Cij

a

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Gravity Model Process• Create Shortest Path Matrix

• How: Minimize Link Cost among Centroids

• Estimate Friction Factor Parameters• How: Function of Trip Length Characteristics by Trip

Purpose

• Calculate Friction Factor Matrix• Convert Productions and Attractions to Origins

and Destinations• Calculate Origin - Destination Matrix• Enforce Constraints on O - D Matrix

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Shortest Path Matrix

• Matrix of Minimum Generalized Cost from any Zone i to any Zone j• Distance, Time, Monetary Cost, Waiting Time,

Transfer Time, etc.. may be used in Generalized Cost

• Time or Distance Often Used• Matrix Not Necessarily Symmetric (Effect of One - Way Streets)

TAZ ID

123456

TAZID 1 2 3 4 5 6

C11 C12 C13 C14 C15 C16 C21 C22 C23 C24 C25 C26 C31 C32 C33 C34 C35 C36 C41 C42 C43 C44 C45 C46 C51 C52 C53 C54 C55 C56 C61 C62 C63 C64 C65 C66

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O - D Matrix Calculation• Calculate Initial Matrix By Gravity Equation, by Trip

Purpose• Each Cell has a Different Friction, Found in the

Corresponding Cell of the Friction Factor Matrix

• Enforce Constraints in Iterative Process• Sum of Trips in Row i Must Equal Origins of TAZ i• Sum of Trips in Column j Must Equal Destinations of TAZ j• Iterate Until No Adjustments Required

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O - D Matrix Example:

Destinations

123456Sum

Origins 1 2 3 4 5 6 SumT11 T12 T13 T14 T15 T16 O1

T21 T22 T23 T24 T25 T26 O2

T31 T32 T33 T34 T35 T36 O3

T41 T42 T43 T44 T45 T46 O4

T51 T52 T53 T54 T55 T56 O5

T61 T62 T63 T64 T65 T66 O6

D1 D2 D3 D4 D5 D6

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Final O - D Matrix• Combine (Add) O - D Matrices for Various Trip

Purposes• Scale Matrix for Peak Hour

• Scale by Percent of Daily Trips Made in the Peak Hour• 0.1 Often Used

• Scale Matrix for Vehicle Trips• Scale by Inverse of Ridership Ratio to Convert Person

Trips to Vehicle Trips• 0.95 to 1 Often Used

• Note: Other Mode Split Process / Models using Discrete Choice may be More Accurate

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OUTPUT = the trip interchange matrix and the shortest path

trees

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Modal Split

• Use a model to convert trips into vehicle trips• Usually public transportation versus private car• Nomographs, Regression methods, Microeconomic

Regression methods• Stated Preference (conjoint measurement)

techniques for hypothetical options• Traveler objective and subjective constraints in

selecting a mode

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OUTPUT = a trip interchange matrix for each mode: car, public transportation, could also be pedestrian but not

usual

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Traffic Assignment

• Fourth Step in UTPS Modeling• Inputs:

• Peak Hour, Passenger Vehicle Origin - Destination (O - D) Matrix

• Network• Travel Time, Capacity, Direction

• Outputs:• Peak Hour Volumes, Estimated Travel Times, and Volume

to Capacity Ratios

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Roadway Performance Functions

• Traffic Flow and Travel Time

Traffic Flow Traffic Flow

Linear Relationship Non-Linear Relationship

Rou

te T

rave

l Tim

e

Cap

acity

Free-FlowTravel Time

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Assignment Methods

• All or Nothing• All traffic from zone i to zone j uses the (initially) minimal

travel time path• Roadway performance not used

• System Equilibrium• Assignment is performed such that total system travel time is

minimized (UPS, Fed-EX).

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Assignment Methods

• User Equilibrium• Travel time from zone i to zone j cannot be decreased by

using an alternate route• Roadway performance used

• Stochastic User Equilibrium• Same as User Equilibrium but accounts for user variability

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Dynamic Assignment Methods

• Assign Traffic by Time of Day

• Estimate origins & destinations by time of day• Apply an equilibrium method• Sometimes incorporate departure time choice• Sometimes incorporate system performance characteristics

See also page 288 of Meyer & Miller, 2001 textbook

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OUTPUT = traffic volumes (cars per hour) on each road, travel times on each link, congestion

levels

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Planning 4-step Procedure Summary

• Land Use and Economics• Trip Generation• Trip Distribution• Modal Split• Traffic Assignment• Main Input=Sociodemographics of study area• Main Output=Traffic volumes and level of service on

roads• Secondary input-output (many depends on

application and software)

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Output from traffic assignment

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Output from traffic assignment

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Case StudySouthern California Association of Governments

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SCAG Model Details & Context• http://www.scag.ca.gov/modeling/2000mv.htm• http://www.scag.ca.gov/committees/pdf/rc/

2004/april/Draft_Final_2004_RTP.pdf• Conformity using DTIM and EMFAC• The EMFAC

http://www.arb.ca.gov/msei/onroad/briefs/emfac7.pdf

• The DTIM• http://aqp.engr.ucdavis.edu/Documents/

newgridmodel.pdf• http://www.icfi.com/Markets/Environment/

enviro2.asp

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Questions?