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CHAPTER 1 Introduction 1.1 Overview of the American Community Survey Census data have long played a central role in transportation planning and analyses. In partic- ular, transportation planners are heavy users of data products concerning population and house- hold characteristics that are derived from the decennial census Long Form, such as the following: Summary File data (SF3 and SF4), tables, and profiles; Public Use Microdata Sample (PUMS) data (1 percent and 5 percent samples); and Census Transportation Planning Package (CTPP) tables (Parts 1, 2, and 3). Beginning with this decade, the Census Bureau's American Community Survey (ACS) will replace the decennial census Long Form as the preeminent source of U.S. population and household characteristics. In previous decennial censuses, residents of one out of every six addresses were asked to complete the Long Form of the census questionnaire, which gathered demographic, socioeconomic, and housing characteristics data, in addition to the base census questionnaire (or census "Short Form"), which gathered constitutionally mandated population counts, along with age, race, and gender information. Beginning in 2010, the decennial census will focus solely on collecting the basic census counts by age, race, and gender. The Census Bureau is now collecting additional population and household characteristics data through the rolling sample ACS. Each year, about 3 million of the U.S. addresses, 36,000 Puerto Rico addresses, and residents of 2.5 percent of group quarters facilities will partici- pate in the ACS, providing data that are more up to date, timely, and probably more accurate than the decennial census Long Form data. The primary cost to the data user is a reduction in sample size and the corresponding need to accumulate data over time and/or across geography. The Census Bureau's stated goals for the ACS are to Provide federal, state, and local governments with an information base for the administration and evaluation of government programs; Facilitate improvement of the 2010 Census by allowing the decennial census to focus on counting the population; and Provide data users with timely demographic, housing, social, and economic statistics updated every year that can be compared across states, communities, and population groups. The Census Bureau began developing the ACS in the mid 1990s after many years of research indicated the potential value of a "continuous measurement" data collection program. During the initial ACS test period between 1996 and 1998, while the program was still somewhat exper- imental, preliminary ACS data were collected for a few test sites. In 1999, the number of test sites was increased to 31 locations, comprising 36 counties and representing a broad range of communities that were selected to provide different combinations 1