Mileage-based user fee initiatives in the United States and abroad show that such fees can lead to more equitable and efficient use of roadways by charging drivers based on their actual road use and by providing pricing incentives to reduce road use. Mileage fees for passenger vehicles, however, continue to face significant public concerns related to privacy as well as cost challenges. Privacy concerns are particularly acute when Global Positioning System (GPS) units are used to track the location of passenger vehicles. Reliable cost estimates for mileage fee systems are not available, but implementing a system to collect fees from 230 million U.S. passenger vehicles is likely to greatly exceed the costs of collecting fuel taxes. Commercial truck user fee systems in Germany and New Zealand have achieved substantial revenues and benefits such as reduced road damage and emissions with fewer privacy concerns, but ensuring compliance in a cost effective manner presents trade-offs. Few commercial truck mileage fee pilots have been conducted in the United States, but efforts in two states suggest such fees pose fewer privacy and cost challenges than passenger vehicle fees. Mileage fee rates could be set to replace or supplement current Highway Trust Fund revenues. GAO calculated average mileage fee rates for passenger vehicles and commercial trucks needed to meet three federal revenue targets ranging from $34 billion (replace current federal fuel tax revenues) to $78 billion (increase spending to maintain existing system conditions and performance). To meet these targets, drivers of passenger vehicles with average fuel efficiency would pay $108 to $248 per year in mileage fees compared to the $96 these drivers currently pay in federal gasoline tax. These fees would affect users' costs differently based on each vehicle's fuel efficiency, because drivers of less efficient vehicles now pay more in fuel taxes than drivers of vehicles with greater fuel efficiency. However, like federal fuel taxes, mileage fees would comprise a small portion of users' overall fuel costs and thus only marginally increase users' overall transportation costs. A mileage fee for commercial trucks could also increase users' costs, particularly for larger trucks that log more miles. In 2000, the Federal Highway Administration (FHWA) estimated that heavy commercial trucks generally pay less in federal taxes than the road damage costs they impose. Adjusting mileage fee rates to account for vehicle road damage costs would increase rates for commercial truck users. However, FHWA's estimates may not reflect current conditions. Setting rates to cover these costs would require updated estimates of vehicles' responsibility for road damage. State departments of transportation (DOT) recognize the need for an alternative funding mechanism to meet future revenue demands, and many would support federal actions to evaluate mileage fees. Few states reported that they are likely to introduce such fees in the next 10 years, but more than half would support federally-led field tests of mileage fees for commercial trucks and electric vehicles. Although few electric vehicles are on the roads today, their numbers are expected to increase, and they do not contribute to the Highway Trust Fund. Without a federal pilot program to evaluate (1) options to more accurately charge commercial trucks and electric vehicles for their road use and (2) the costs and benefits of such systems, Congress lacks critical information to assess whether mileage fees for these vehicles could be a viable and cost-effective tool to help address the nation's surface transportation funding challenges.