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CHAPTER 9 Travel Demand Modeling Analyses Using ACS Data This section describes travel demand modeling as a common application of census data. Section 9.1 defines travel demand modeling and describes how census data can be used to support it. This section also provides some examples of uses of census data for this purpose. A more detailed list of specific uses is provided at the end of this section. Section 9.2 describes some benefits and limitations of shifting from census to ACS data related to travel demand modeling. Section 9.3 provides two case study examples. The first case study shows how to estimate an auto ownership model using ACS PUMS data. The second case study describes how ACS data may be used in the validation of a trip distribution model. Finally, Section 9.4 details the specific uses of census data for travel demand modeling. 9.1 Travel Demand Modeling Travel demand modeling consists of a variety of mathematical models developed to support long-range transportation plans and policy planning analyses. Transportation planners have used decennial census data for different components of travel demand modeling, including trip generation, trip distribution, mode choice, traffic assignment, demographic and auto ownership models, and microsimulation. The specific ways in which census data can assist in travel demand modeling are described next. 9.1.1 Trip Generation Traditional trip generation models relate the number of trips produced and attracted in TAZs to the characteristics of those zones. Census data are generally the best source of zonal estimates. 9.1.2 Model Input for the Base Year Transportation planners rely heavily on census data as a primary source of socioeconomic and demographic data needed as base-year input to travel demand models. Almost all trans- portation planners contacted during this research use census data in this context and have updated (or are in the process of updating) their travel demand models to include the 2000 socioeconomic data. Where CTPP data were still unavailable at the time that the plan- ners provided their opinions, these planners were often using Summary Files 1 and 3 to sup- port modeling applications. Many MPOs participated in the TAZ-Update Program to define/transfer their local TAZ structure into TIGER/Line 2000. Some MPOs aggregate block or block group level data to define TAZs. CTPP 2000 provides data at the TAZ, census tract, and--in some cases--block group geography. MPOs are able to use these data easily in their models. 141
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142 A Guidebook for Using American Community Survey Data for Transportation Planning 9.1.3 Trip Generation Rates Since data on trip frequency per household or worker are not available from the census, trip generation models cannot be estimated using census data. However, observed work trip travel patterns could be used to calibrate work trip generation models. Trip attraction models might be more difficult to calibrate and validate due to various issues associated with the way the cen- sus estimates employment. 9.1.4 Trip Distribution Aggregate calibration of friction factors used in gravity work trip distribution models is being done using observed flows from the census, by monitoring average commute and commute time frequency distribution.99 9.1.5 Work Trip Mode Choice Modeling Mode choice models cannot be estimated using census data, but the data can be used to cali- brate and validate existing work-based mode choice models. 9.1.6 Traffic Assignment Modeling Census travel time data are used to calibrate and adjust speeds and travel times in traffic assignment models. 9.1.7 Demographic and Auto Ownership Models Estimation and validation of demographic (e.g., household income distribution models, distribution models for households by number of workers/persons/vehicles available in house- hold) and auto ownership models are being performed using census data. For example, disaggregate models are being estimated using PUMS data.100 Aggregate validation of those models could be done using CTPP Part 1 (CTPP also can be used for aggregate estimation of models). 9.1.8 Microsimulation In addition to the traditional modeling steps, census data are being used for more advanced model components as well, such as using PUMS data for population synthesis for microsimula- tion models. 9.1.9 Examples of Use This section provides some examples of presenting travel demand modeling analyses. Figure 9.1 shows a home-based work trip length frequency distribution,101 and Figure 9.2 shows out-of-county 99 Examples of DOTs/MPOs where trip distribution calibration efforts were done using Census data are: Indiana DOT, Vermont Agency of Transportation, Mass Highway, Chicago Area Transportation Study (personal corre- spondence). 100 Travel Model Improvement Program, U.S. DOT, "Model Validation and Reasonableness Checking Manual." See http://tmip.fhwa.dot.gov/clearinghouse/docs/mvrcm/ch1.stm. November 4, 2004. 101 A. Noelting, 2005. "U.S. Census, CTPP, and NHTS Data Used in the Des Moines Area MPO's Travel Demand Model." See www.fhwa.dot.gov/ctpp/sr0105.htm.
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Travel Demand Modeling Analyses Using ACS Data 143 Figure 9.1. Home-based work trip length frequency distribution. Figure 9.2. Out-of-county commute map.