10. trip generation

Norman W. Garrick

Transportation ForecastingWhat 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’

Norman W. Garrick

The Four Step Model

• Trip GenerationEstimates the number of trips from given origins and destinations

• Trip DistributionDetermines the destination for each trip from a given origin

• Mode ChoiceDetermines the mode choice for each trip

• Route AssignmentDetermines the specific route for each trip

Norman W. Garrick

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

1

23

4

5

6

87

Norman W. Garrick

Trip GenerationDeveloping and Using the Model

Calibrated Model

Relating trip making to socio-economic and land use data

Estimated Target year

socio-economic,land use data

Predicted Target year No. of Trips

Survey Base YearSocio-economic, land use

And Trip making

Norman W. Garrick


The trip generation model typically can take the form of

No. of trips = Function (pop, income, auto ownership rates)

The model is developed and calibrated using BASE year data

Norman W. Garrick

Trip GenerationDeveloping the Model

Trip Generation models are often developed from travel surveys. These surveys are used to determine the trip making pattern for a sampling of house holds in the area. This trip making pattern is then related to land use (nominally) and socioeconomic factors that are considered to affect travel patterns

Common socioeconomic factors considered include population, income, and auto ownership rates

Norman W. Garrick

Trip GenerationTrip Purpose

Often separate predictions are mode for different type of trips since travel behavior depends on trip purpose

In other words different models must be developed for each trip type

The category of trip types commonly used include• Work trips• School trips• Shopping trips• Recreational trips

Norman W. Garrick

Trip GenerationExample of a Trip Generation Model

One way of presenting the trip generation model developed from a survey is as a cross-classification table

Norman W. Garrick

Trip GenerationSurvey and Model

Calibrated Model



And Trip making

Norman W. Garrick

Trip GenerationCross-classification Model

Total Home-Based-Non-Work Trip RatesPersons per Household

Type of Area

Vehicles per HH

1 2,3 4 5+

High Density

0 0.6 2.1 4.6 7.0

1 1.5 3.0 5.5 7.9

2+ 1.8 3.4 5.9 8.3

Low Density

0 1.0 2.5 5.0 7.4

1 1.9 3.5 6.0 8.4

2+ 2.3 3.9 6.4 9.0

Norman W. Garrick


Calibrated Model





And Trip making

Norman W. Garrick

Trip GenerationBase Year Demographics

Number of Households in Target YearPersons per Household

Type of Area

Vehicles per HH

1 2,3 4 5+

High Density

0 100 200 100 100

1 200 300 200 1002+ 100 200 100 200

Low Density

0 50 100 100 100

1 100 200 100 1002+ 100 100 100 10

Norman W. Garrick


Calibrated Model




Predicted Target year No. of Trips


And Trip making

Norman W. Garrick

Trip GenerationBase Year Demographics

Number of Trips in Target Year for Each HH Type

Persons per Household

Type of Area

Vehicles per HH

1 2,3 4 5+

High Density

0 60 420 460 700

1 300 900 1100

790

2+ 180 680 590 1660

Low Density

0 50 250 500 740

1 190 700 600 8402+ 230 390 640 90

Number of Trips = trip rate*no. of HH = 0.6 * 100 = 60 Total trip = 13,080

Norman W. Garrick

Trip GenerationPlanning for the Future and

Uncertainties

In an earlier lecture we talk about the uncertainties associated with making prediction for the future and the importance of not treating predictions as if they are set in stone but rather as a guide to help in decision making

In considering the ‘trip generation’ process it is important to understand some potential sources of uncertainties

Norman W. Garrick

Trip GenerationSources of Uncertainties in Predicting Number of Trips

Significant errors can creep into the trip generation process in another of places including

The model developmentThe Prediction of future demographicsThe use of the model for predicting the future

Norman W. Garrick

Trip GenerationModeling Land Use is one Major Source

of Uncertainty

• The modeling of the different types of land use and their effect on travel pattern is usually relatively weak

• Land use and transportation changes alter the relationship between the number of trips and the socio-economic factors. But the trip generation process typically assume that this relationship is constant over the period of the study

Norman W. Garrick

Trip GenerationDemographics and Trip Making Factors

affected by Land Use

The land use pattern may affect

Car ownership rates Household size and composition Number of daily trips Mode of trips Length of trips

Norman W. Garrick

Trip GenerationWhat 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

Norman W. Garrick

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)

Norman W. Garrick

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)

Norman W. Garrick

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 endsNon-residential land use ATTRACTS trip ends

This is a useful distinction because of how trip generation models are typically developed

Norman W. Garrick

Modeling Productions and Attractions

Trip generation models typically model separately, i) residential trip production, ii) non-residential trip attractions

1

Non-residential

Residential

For example, Trip Ends for Zone 1 would be reported as 1. 1000 Production Trip Ends2. 500 Attraction Trip Ends

This approach works for home based trips (HB). But falls apart when we start to consider non-home based trips (NHB). Special techniques are developed to deal with the relatively small number of NHB that occurs.

Norman W. Garrick

Trip Rate Analysis Method of Trip Generation

Trip-Rate AnalysisTrip rate is estimated on characteristics of the trip generators with in the zone. Production rates are determined using the characteristics of the residential land uses and attraction rates using the characteristics of the nonresidential land uses

Example

The characteristics of the trip generator is given in 1000 SQ. FT.And the trip generation rate for each generator is given as TRIPS PER 1000 SQ. FT.

For exampleResidential: Total 1000 Sq. Ft. = 2744 1000 sq. ft., Trip Gen. Rate = 2.4 trips/1000 sq.ftTOTAL NO. of TRIP from residential land use = 2744*2.4 = 6586 Trips

This method of trip generation is often used to do site impact studies

10. trip generation

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