Since FY2008, federal highway user taxes and fees have been inadequate to fund the surface transportation program authorized by Congress. Although the 2015 surface transportation act addressed the revenue shortfall through FY2020 by authorizing the use of general funds for transportation purposes, the Congressional Budget Office (CBO) projects that after FY2020 the gap between dedicated surface transportation revenues and spending will average $20 billion annually. The search for revenue to fill this gap may revive congressional interest in tolling as a means of financing transportation projects without federal expenditures. Although states are free to impose tolls on roads, bridges, and tunnels that have been built and maintained without federal assistance, federal law limits the imposition of tolls on existing federal-aid highways, especially on the Interstate Highways. This report explains current federal policies governing tolling and discusses issues related to increasing the use of tolls as a source of revenue for surface transportation projects. In some states, mostly in the Northeast and the mid-Atlantic region, many of today's highways were originally toll roads, often built and operated by private investors. While tolling often made it possible to build or improve roads at minimal cost to taxpayers, many of these roads failed due to overly optimistic revenue expectations, inability to attract sufficient investment to pay for improvements, competing capacity, and toll avoidance and the related cost of enforcement. Over time, toll roads came to be regarded as obstacles to the free flow of commerce. When it established the forerunner of today's federal-aid highway program in 1916, Congress emphasized the principle that roads should be free. Section 1 of the Federal Aid Road Act (39 Stat. 355) provided that "all roads constructed under the provision of this Act be free from tolls of all kinds." The Oldfield Act of 1927 (44 Stat. 1398) opened the door to tolls by permitting the use of federal funds to build toll bridges as long as they were operated by the states or their political subdivisions. However, the federal Bureau of Public Roads continued to oppose the use of federal funds on toll roads. Consequently, when states, mainly in the Northeast, undertook expressway construction in the decade after World War II, they built toll roads without federal aid. By January 1, 1955, there were 1,239 miles of completed "arterial toll roads" in the United States, another 1,382 miles were under construction, and 3,314 miles were being planned or studied. Many of these roads were on routes of the planned Interstate system. Although the Bureau of Public Roads supported the building of new Interstate Highways as free roads, it did recommend that existing toll roads that met its engineering standards and followed the routes of proposed Interstate Highways be incorporated into the new network. The tolling prohibition was reiterated in the Federal-Aid Highway Act and Highway Revenue Act of 1956 (P.L. 84-621; 70 Stat. 374), which authorized 13 years of funding for construction of the Interstate Highway system, created the Highway Trust Fund (HTF) as the source of federal funds for state road construction, and raised tax rates on motor fuels to help fund it. The fuel and other highway taxes that were now dedicated to the HTF were seen as a close proxy for a user-payer system of financing federal-aid roads. The increased flow of federal funds, heavily weighted toward the Interstate Highways, effectively stopped the development of new toll roads by the states. Thirty-five years later, the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA; P.L. 102-240 ) opened non-Interstate federal-aid highways to tolling, but allowed existing roads or bridges to be tolled only after being reconstructed. This effectively linked tolling to capacity additions or road improvements. Both the 1998 Transportation Equity Act for the 21 st Century (TEA-21; P.L. 105-178 , as amended by P.L. 105-206 ) and the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users ( P.L. 109-59 ; SAFETEA) allowed tolling on high-occupancy vehicle (HOV) lanes, established pilot projects for tolling of a limited number of Interstate system routes, and allowed limited use of tolls that vary according to the level of traffic, known as congestion pricing. The Moving Ahead for Progress in the 21 st Century Act (MAP-21; P.L. 112-141 ), enacted in 2012, reinforced the encouragement of tolls on HOV lanes and congestion pricing. It allowed new Interstate system routes or route extensions to be built as toll roads, but continued to block tolling of most existing Interstate Highway lane capacity. MAP-21 retained two pilot programs, one encouraging the use of pricing to control congestion and another allowing Interstate route segments in three states to be converted to tolling as part of their reconstruction. The Fixing America's Surface Transportation Act (FAST Act; P.L. 114-94 ), enacted in December 2015, clarified that public authorities generally, as opposed to solely state agencies, may impose tolls on single-occupant vehicles using HOV lanes. It modified the TEA-21 pilot program allowing existing Interstate Highway segments in three states to be subject to tolls to finance reconstruction by providing that federal approval lapses if the selected states have not started construction within three years of approval. The law also included two provisions related to the setting of rates. One mandates that intercity buses that serve the public have the same access to toll roads and pay the same rates as public transportation buses. The other requires public authorities operating high-occupancy toll lanes (HOT lanes) on the Interstate system to consult with the metropolitan planning organizations concerning the placement and amount of tolls on the facility. Other than these changes, the FAST Act continued the general federal government policy of not regulating toll rates. Table 1 , below, briefly describes active federal tolling programs. An important attribute of federal tolling policy is that all conversions of existing federal-aid highways, bridges, or tunnels to toll facilities require that the facility be reconstructed, restored, rehabilitated, or replaced (unless the conversion occurs under the Value Pricing Pilot Program). The decision to convert a free facility to a tolled facility must be made prior to completion of the qualifying reconstruction project. According to the Federal Highway Administration (FHWA), once physical construction is completed it is too late to make the decision to toll, unless an additional qualifying reconstruction or rehabilitation project is undertaken. Whether it is built or operated by a government agency or by private investors, a toll road must have sufficient traffic willing to pay a high enough toll to cover construction, maintenance, and toll collection costs if it is to be financially successful. Most roads on the federal-aid system are not likely to pass that test. In rural areas, highways often do not have enough traffic to cover the cost of building toll-collection infrastructure and collecting tolls. While urban roads typically have more traffic, they may not be able to generate sufficient toll revenue to make the facilities self-sustaining. Some publicly owned toll roads have been financially successful, generating sufficient revenue to pay for capital improvements and operations, and, in some cases, to contribute to the cost of other highway activities and even to public transportation activities. Other public toll facilities, however, have struggled. One recent example is the 8 miles of express toll lanes built by the Maryland Transportation Authority on I-95 north of Baltimore. In the seven months following their opening in December 2014, the toll lanes produced $6 million in revenue before operating expenses, far from enough to cover the cost of financing the $1.1 billion project. The state government has had to use other funds to make up the difference. Recent federal policy has encouraged the use of tolling to attract private investment into highway and bridge construction, but a number of private toll roads have proven to be financial failures. The Pocahontas Parkway, an 8.8-mile-long toll road near Richmond, VA, that opened in 2002, has persistently been unable to service debt due to low traffic volumes; in June 2012, its private operator wrote off the entire value of its investment in a 99-year concession, and eventually transferred the lease to a new operator. SH-130, a 90-mile, four-lane toll road near Austin, TX, has had much lower traffic volumes than forecast when it opened in 2012, and the Texas Department of Transportation ended up subsidizing truck tolls in an effort to help make the privately owned project viable. Despite the subsidy, in March 2016 the toll road operator, SH 130 Concession Co., and two affiliates filed for Chapter 11 bankruptcy protection. Other toll roads that have sought bankruptcy protection include the South Bay Expressway in San Diego, CA, and the Indiana Toll Road. All of these financial failures were public-private partnerships (P3s) that were formed based, at least in part, on overly optimistic forecasts of the revenue that would be provided by tolls. Their widely publicized difficulties have made investors more cautious about projects reliant on toll revenue. In some cases, private-sector investors have conditioned their participation in P3s on "availability payments," regular payments from the sponsoring government entity to the private entity. This reduces the risk to the private entity, but leaves the public entity at risk if toll revenue falls short of expectations. Using tolls to support transportation expenditures may be a comparatively inefficient form of funding because of high administrative costs. Collecting federal motor fuels taxes is estimated to cost about 1% of the amount collected. The process is administratively simple, because nearly all the federal fuels taxes are collected at the terminal "rack" from only 850 registered taxpayers nationwide, rather than at a large number of retail gasoline stations. The small number of collection points also facilitates enforcement. The administrative costs of toll collection appear to be significantly higher than the cost of fuel-tax collection.