The U.S. Government Accountability Office (GAO)’s Highway Trust Fund: Pilot Program Could Help Determine the Viability of Mileage Fees for Certain Vehicles report notes that federal funding to build and maintain the nation’s highways and bridges comes primarily from highway users through federal fuel taxes. These revenues have eroded due to improvements in vehicle fuel efficiency and other factors contributing to shortfalls in the Highway Trust Fund. Experts have proposed alternative means of raising revenues by charging drivers fees based on their miles traveled. Several states have tested systems that gather vehicle mileage and location data, which has raised privacy concerns. GAO examined (1) the benefits and challenges of mileage fee initiatives in the United States and other selected nations, (2) mileage fee rates necessary to replace and supplement current Highway Trust Fund revenues and the effect these fees would have on users’ costs, and (3) state DOTs’ views on future revenue demands and mileage fees. GAO reviewed five domestic pilot projects and programs in Germany, New Zealand, and the Netherlands; modeled mileage fees for passenger vehicles and commercial trucks; and surveyed 51 state DOTs.