"The federal government has provided some form of highway funding to the states for roughly 100 years. The major characteristics of the federal highway program have been constant since the early 1920s. First, most funds are apportioned to the states by formula and implementation is left primarily to state departments of transportation (state DOTs). Second, the states are required to provide matching funds. Until the 1950s, each federal dollar had to be matched by an identical amount of state and local money. The federal share is now 80% for non-Interstate System road projects and 90% for Interstate System projects. Third, generally, federal money can be spent only on designated federal-aid highways, which make up roughly a quarter of U.S. public roads."
CRS Report for Congress, R42793