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132 A Guidebook for Using American Community Survey Data for Transportation Planning the decennial census Short Form data. If variables such as auto ownership or income are used in the expansion (as is usually the case), they are obtained from census products based on the Long Form (such as SF3 or CTPP). Census data also are used in validating travel surveys by providing information on the distri- butions of various socioeconomic, demographic, and journey-to-work characteristics that can be used for the validation of an expanded survey. 8.1.2 Specific Examples of Use Almost all household travel surveys have been expanded using census data. Among the sam- ple expansion variables that have been used in some recent surveys are the following: Year 2000 home interview survey conducted by the Metropolitan Council, St. Paul-Minneapo- lis, Minnesota, was expanded by number of households, household size, and vehicle availability. Year 2000 home interview survey conducted by Memphis MPO was expanded by auto avail- ability, income, and household size. Year 1990 household travel survey conducted by the Chicago Area Transportation Study was expanded by household size and vehicle availability. Year 1990 San Francisco Bay Area household travel survey was expanded by superdistrict of residence, household size, vehicle availability, and tenure. Household travel survey conducted by the Central Transportation Planning Staff, Boston, was expanded by vehicles available, household income, and workers in household. The household survey conducted by the Denver Regional Council of Governments was expanded using household size and income variables at the county level from the census. Many household travel survey efforts also have relied on census data to confirm and validate the results of the surveys. One example of household travel survey validation is the validation done at the Metropolitan Council (St. Paul-Minneapolis,), where CTPP Part 1 data were used to check the mode split from the home interview survey done concurrently with the census, and CTPP Part 3 data were used to validate the survey home-based-work trip distribution. 8.2 Benefits and Limitations of ACS for Survey Development and Analysis This section summarizes the perceived benefits and limitations of using ACS data for survey design, expansion, and validation. Transportation planners who were asked about the potential use of ACS estimates to support travel survey efforts envisioned that because measurement is done continuously with ACS, it will be easier to conduct surveys and expand them using more recent data at any point in time (e.g., mid-decade) since they can be expanded by the large-area data from ACS. With the decen- nial census, the data used for sample design and expansion are often either extrapolated or out of date. Although ACS can potentially provide more current data for survey expansion, the use of multiyear averaged variables for survey expansion could pose problems. It is expected that the multiyear ACS estimates for household income will be particularly difficult to interpret. If house- hold characteristics change over a five-year period as household income is likely to, then ACS average data might be inconsistent with other demand-side data sources and with household travel surveys conducted during fixed periods of a few weeks. In addition, the higher sampling error associated with ACS estimates will increase the level of uncertainty in the development of household survey expansion targets.