3. Data Related Issues
3.1 Current versus Constant Dollars
GTFS includes revenue and expenditure statistics expressed in current and constant dollars. The statistics in current dollars are converted to the 2005 constant dollars by using the chained-dollar price index. Chained-dollar (or constant dollar) estimates provide dollar values of revenue and expenditures that are adjusted to remove the effects of inflation. They are computed by dividing the current dollar estimates by a chain-type price index and multiplying by 100.
The chain-type price index of government transportation consumption and gross investment is used as the deflator for this purpose. The same index is used for Federal, State, and local government revenue and expenditure. The data for the price index are obtained from BEA’s National Income and Product Accounts Tables.
3.2 Key Data Sources
GTFS provides updated and revised government transportation revenue and expenditure statistics from 1995 to 2009. Below is a list of the principal data sources that make periodic revisions, corrections, or updates to their data, requiring corresponding revisions to the historical data
series of GTFS. More detailed description of data sources are presented in Appendix B.
Public Budget Database: The Federal government transportation finance data are primarily obtained from OMB’s Public Budget Database. To improve the accuracy of data, OMB makes periodic revisions and corrections to the data in the database. The current GTFS report reflects all the revisions or changes made by OMB.
Highway Statistics: FHWA releases annual data of Federal highway receipts and expenditures reported in table FA-5 of the Highway Statistics report. The annual release and revisions affect the data on Federal highway receipts and expenditures in GTFS.
National Transit Database (NTD): NTD provides detailed information on outlays and receipts of transit agencies on an annual basis. The State and local government transit expenditures and revenues are obtained from this data source. For years 2001 and previously, revenue data were reported in NTD by different sources of funding of transit expenditure. The revenue reported during this period was not necessarily earned during the reporting period. Instead, the funds may have included revenue earned in prior years. Beginning in 2002, the data reported in NTD have been enhanced to cover revenue earned by transit agencies, which was not provided in earlier years. Consequently, the data in GTFS have been revised to reflect the actual revenue earned as opposed to revenue expended during the period.
Airport Financial Report: State and local government airport revenue is based on information from FAA’s Airport Financial Report. The Airport Financial Reporting Program gathers and disseminates airport financial information on the FAA Web site and makes continuous revisions and updates to the data, requiring corresponding revisions in the GTFS report.
Corporate income taxes: The income tax payment of business establishments involved in providing transportation services are taken from BEA. Similar to the chained price index data, the agency revises the income statistics on an annual basis. These revisions are also reflected in the GTFS data.
3.3 Data Gaps and Limitations
There are several data gaps that exist in GTFS that users need to take into account when interpreting the data. Those data gaps include the following:
State-specific statistics: The GTFS 2011 provides Federal, State, and local government transportation revenue and expenditures. While the data for State and local governments are reported at the aggregate level, data are lacking for individual State and local governments. That is due to the fact that no consolidated and consistent data are available from a single source that provides comprehensive financial statistics by State for all modes of transportation. Additionally, Federal grants are only available at the aggregate level by mode.
Water transportation expenditure: All Federal outlays for water transportation, including outlays in the form of direct Federal spending or grants to State and local governments, are not covered in the data. In particular, Federal outlays for civilian transportation related programs or activities of the United States Army Corps of Engineers are not included.
State and local expenditures for railroads, pipeline, and general support: The data in GTFS for railroads, pipeline, and general support include the part of State and local expenditures funded by Federal grants only. The components of outlays that may be funded by other funding sources of State and local governments are not covered.
- Property taxes: Not all receipts from transportation-related property taxes are included in this report. For example, personal property taxes on motor vehicles and taxes on motor carriers based on assessed value of property are not included in State and local highway revenue.