A concern in using the ACS 5-year period estimates for traffic analysis zones is that the 60-month averages that underlie the estimates may obscure short-term changes in commuting patterns that occur in response to marked changes in the local economy or the transportation infrastructure. To address this concern, transportation planners in large cities and metropolitan areas can examine 3-year or 1-year period estimates for the area as a whole, for PUMAs, and, in some cases for smaller cities and towns. Analyses of these estimates can provide an overall sense of changes in commuting modes and times to work that can inform assessments of the usefulness of the 5-year period estimates.
Precision is also a very serious concern for 5-year period TAZ estimates. Statistical mapping techniques may help transportation planners extract useful information from the estimates in some instances. For example, by geographically displaying such variables as mode of transit to work, where workers live, and where workers work on maps of transportation routes, places of employment, and other local features, planners may see patterns that suggest how to combine TAZ estimates to produce meaningful larger areas that have more precise estimates. (Such statistical mapping techniques may help users in other fields extract value from ACS 5-year period estimates for census tracts and block groups.)
The usefulness of 5-year period TAZ estimates also depends importantly on two other factors: (1) procedures that the Census Bureau uses for imputing missing responses and (2) decisions it makes regarding the data that can be provided while protecting confidentiality. Regarding imputation, the Census Bureau needs to engineer its data processing so that imputations for missing responses to commuting items can be made at the outset at the block level. In the long-form-sample processing, imputations for these items were made initially at the city level and only subsequently, in the CTPP, carried out at the block level.
Regarding confidentiality, the Census Bureau needs to consider carefully the added confidentiality protection afforded by 5-year averages compared with point-in-time estimates. The added protection results from the fact that many people change one or more characteristics of interest over a 5-year period, such as place of work, occupation, place of residence, commuting mode, etc. Consequently, the risk of reidentification of a specific individual in 5-year aggregations is reduced. Taking account of this added protection should enable the Census Bureau to release sufficient information on commuting (and other topics) to be useful at the level of traffic analysis zones, block groups, and census tracts (see Section 4-D.1).