Here is information I provided to Chandler Felt (original query). Federal Highway Administration sponsored research conducted comparing the ACS (1999 -2001) test data to the 2000 decennial data. We used 6 counties, and later, compared tract level data for 2 counties. This research was conducted on-site at the CB offices in Suitland, with access to the microdata. (Wende Mix, formerly at Westat) To summarize the file attached, the average travel times were significantly different between the ACS and the decennial. Generally speaking, the average travel time in Census 2000 are from .8 min to 1.8 minutes LONGER than the ACS reported times. As Phil notes, when examining the travel time distributions, the ACS curves are shifted to the left (shorter times) compared to decennial. (my file that has bar charts that show the distribution is too big for the listserv!) Pima County ACS 25.2 Decennial 26.7 * San Francisco 33.3 34.6 * Broward, FL 28.7 30.5 * Lake, IL 34.6 34.0 * Flathead, MT 20.6 21.4 Bronx, NY 48.0 49.2 * * = significantly diff When examined spatially (by tract) and by mode, there was no pattern found in our limited research to explain the difference. We did not compare by month or season, but some of us suspect some seasonality differences. Another factor may be that the decennial is more likely to capture longer distance commutes. We have not yet been able to compare this, but have an NCHRP project that includes this question. Other researchers believe that the use of field interviewers are resulting in overall higher quality data. We agree that it is not wise to compare the "current" ACS results directly with decennial 2000, since it appears that methodological differences are making a significant difference in this variable. Elaine Murakami FHWA Office of Planning 206-220-4460