"The prohibition of tolling of federal-aid highways dates back to the Federal Road Act of 1916 (39 Stat. 355). Subsequent legislation modified the prohibition to the point where now the only significant part of the federal-aid highway system under the toll prohibition is the Interstate Highway System, comprising approximately 47,000 miles of the roughly 1-million-mile federal-aid highway system. Congress, in approving the Federal-Aid Highway Act and Highway Revenue Act of 1956 (P.L. [Public Law] 84-621; 70 Stat. 374), rejected the use of tolls to finance construction in favor of creating a highway trust fund supported by dedicated fuel taxes. However, 2,102 miles of tolled expressway segments were incorporated into the Interstate Highway System in 1957, and these segments are not covered by the tolling prohibition. In recent years, the revenues flowing into the highway trust fund have been insufficient to maintain even current levels of federal funding for highways. Political resistance to raising the federal fuels tax is high. The fuel taxes dedicated to the highway trust fund, currently 18.3 cents per gallon of gasoline and 24.3 cents per gallon of diesel fuel, were last raised in 1993. Historically, interest in toll financing has increased during periods of constrained federal funding. Since the Interstate highways make up nearly all federal-aid highway segments that are still under the tolling prohibition, advocates of expanded use of tolling focus their efforts on giving states more flexibility to impose tolls on the Interstates within their borders."
CRS Report for Congress, R42402