The list below contains the major limitations of the database that users should take into consideration:
- Government revenue in this report excludes money collected by the government from transportation sources but not used for transportation purposes. A typical example of such type of revenue is the highway trust fund receipts designated for deficit reduction. Transportation revenue also excludes money collected by the government from non-transportation sources but spent for transportation purposes, such as general fund money used for transportation purposes. On the other hand, government transportation expenditure includes all money spent by the government for transportation purposes regardless of the sources of funding. For example, government budget appropriations for offices, agencies, and programs of the U.S. Department of Transportation are counted as transportation expenditure, whether they are funded directly from transportation-related trust funds or though general fund money of the Treasury.
- Federal government data are compiled for the federal fiscal year, which begins on October 1, while state and local data are for fiscal years that generally start in July, except for four states with other starting dates (Alabama and Michigan in October, New York in April, and Texas in September). While this may create a small error in totals for any given year, the data are suitable for illustrating trends in public transportation finance.
- State and local government data for census years are full counts and are not subject to sampling errors, whereas the data for non-census years are estimates from annual surveys of the Bureau of the Census, which are subject to sampling errors.
- The database is limited to civilian transportation, including civilian activities of the U.S. Army Corps of Engineers (USACE); such as construction and maintenance of channels, harbors, locks, and dams; and civilian transportation-related activities of the U.S. Coast Guard (USCG).
- State and local transportation-related property taxes are not covered due to lack of data. For example, personal property taxes on motor vehicles and taxes on motor carriers based on assessed value of property are not included in the state and local highway revenues.
- Not all transportation-related activities by government agencies outside of the U.S. Department of Transportation are covered due to the lack of data. For example, expenditures of the Environmental Protection Agency (EPA) for transportation-related pollution programs are not covered, because this portion of the spending is not separately reported in the budget.