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Appendix A â A Note on commuting data from ACS vs. LEHD The methodology recommended for estimating the demand for passenger transportation for commuting travel from rural counties to urban centers is the use of a function for estimating the share of commuting trips that would use a passenger transportation service. Data on commuting patterns, and in particular the number of persons commuting from one area to another, is available from the Census Bureau from two sources â the American Community Survey and the Longitudinal Household- Employment Dynamics data. A discussion of the use of these datasets can be found in NCHRP 08-36, Task 098 Improving Employment Data for Transportation Planning. This report can be found at: http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP08-36(98)_FR.pdf Of relevance for the estimation of demand for travel between rural counties and urban centers is the finding that âthe LEHD-OTM is an excellent source of data for constructing or validating a detailed OD table of home-to-work flows between geographic areas that can range from as small as individual Census Blocks to entire states. Unlike sample-based surveys (such as the CTPP), the LEHD-OTM provides a (nearly) complete enumeration of home-to-work flows covering over 90 percent of all workers and employers in the United States. As such, it includes many more OD pairs containing low frequency home-to-work flows than are collected through sampled data.â Because the flows from rural counties to urban centers are likely to be small compared to flows within urban areas, the use of LEHD-OTM as the basis for estimation of the demand for passenger transportation for commuters is recommended. The text in the remainder of this Appendix is taken directly from NCHRP 08-36. Longitudinal Employer Household Dynamics The LEHD Program is based on a negotiated partnership arrangement between the Census Bureau and each state ESA. This partnership has evolved over a period of more than a decade, with the last few states joining as recently as 2010. Under the LEHD Program, the Census Bureau obtains a copy of the same enhanced microdata files that used to produce the QCEW, and merges these data with additional administrative data on individual workers that the Census Bureau collects from other federal agencies. The data are merged internally within the Census Bureau and are subjected to a series of âdisclosure proofingâ procedures to prevent release of confidential information on the identity of an individual worker or employer. The integrated employer-worker data is then made available through two different databases â the QWI and OTM. OnTheMap The LEHD-OTM is a unique database that combines information on both the residence and workplace locations of workers at a level of geographic resolution (Census Block) that is most useful for transportation planning and travel demand modeling applications. Unlike the QCEW and QWI, which are employer-based, the LEHD-OTM is more worker-based, providing information on where workers in specific socio-demographic categories (i.e., age, income) and industry sectors live and work. The LEHD- OTM is published annually, approximately one year following the reference year for which the data are collected.
Comparison of LEHD-OTM and CTPP Databases The LEHD-OTM work flow data were compared with journey-to-work trip data collected through the Census Transportation Planning Products (CTPP) in an effort to further evaluate the strengths and limitations of LEHD-OTM data for transportation planning applications. The comparisons included county-to-county flows using the LEHD-OTM, CTPP 2000, and the CTPP 2006-2008 3-year summary databases, and Tract-to-Tract flows for two metropolitan areas using the LEHD-OTM and CTPP 2000 databases. Table ES.1 highlights key differences among the three databases. Key Differences in Employment Data Available from the CTPP and the LEHD-OTM Databases Comparisons at the county and Census Tract levels showed that both the CTPP 2000 and CTPP 2006- 2008 databases include more total home-to-work flows than the LEHD-OTM database, but distribute those flows among a significantly smaller number of OD pairs. This results in significantly higher average flow rates for each non-zero OD pair in the CTPP databases, but many more OD pairs (with lower average flow rates) in the LEHD-OTM database. The comparisons suggest that the LEHD-OTM data captures many more of the low frequency OD pairs than either the CTPP 2000 or CTPP 2006-2008 databases. The CTPP databases are derived from a sample of U.S. households, which are then expanded to the universe of all households based on demographic factors. One consequence of this methodology is that OD pairs with a low frequency of home-to-work trips that are sampled in the CTPP get weighted more heavily, while low frequency OD pairs that are not sampled are assumed to have no home-to-work flows. The result is a âlumpyâ distribution of flows that becomes even more âlumpyâ as the sample size decreases (i.e., from the CTPP 2000 to the CTPP 2006-2008). The county-to-county and Tract-to-Tract flows from the LEHD-OTM were also compared against the CTPP databases with respect to travel distance between OD pairs. While the distributions are generally
similar in shape, a larger percentage of flows in the LEHD-OTM are longer distance (i.e., 25+ miles) than in the CTPP databases. While some of this difference can be attributed to the large number of longer distance, low-frequency OD pairs identified in the LEHD-OTM that were not sampled in CTPP databases, other contributing factors may include (1) the absence of self-employed workers in the LEHD-OTM, who are more likely to work at home or at workplaces closer to home than other employment categories; and (2) employers with multiple worksites who file incomplete multiple worksite reports (MWR) with a state ESA. Workers could therefore be misallocated to an employerâs primary worksite, rather than a secondary worksite that is closer to their residence. LEHD-OTM data were compared to CTPP databases with respect to both employment destinations and residence-to-workplace flows, both at the county and Census Tract levels of geography. The findings from these comparisons were inconclusive as to whether inaccuracies in MWR reporting leads to serious inaccuracies in employment site locations. While significant differences in work destinations were clearly observed between the databases, many of these differences could be attributed to missing employment categories in the LEHD-OTM, the absence of flows between low frequency OD pairs in the CTPP data, or temporal differences in when the data was collected (i.e., 2000 CTPP vs. 2006 LEHD-OTM). Additionally, potential indicators of locational inaccuracies attributable to multi-site employers (e.g., higher work flows to locations housing state capitals or headquarters for large corporations) were not consistent from one site to another. The LEHD-OTM should not be viewed as an alternative to either household travel surveys (including the CTPP) or to employer-based surveys (such as the QCEW), but rather as a complement to both types of data. The LEHD-OTM database does not contain information about the work trip itself; there are no attributes describing the choice of mode, route, travel and departure times, or costs for the trip to work. However, the LEHD-OTM is an excellent source of data for constructing or validating a detailed OD table of home-to-work flows between geographic areas that can range from as small as individual Census Blocks to entire states. Unlike sample-based surveys (such as the CTPP), the LEHD-OTM provides a (nearly) complete enumeration of home-to-work flows covering over 90 percent of all workers and employers in the United States. As such, it includes many more OD pairs containing low frequency home-to-work flows than are collected through sampled data.