As states across the nation are considering proposals for transportation funding within their borders, two new reports are considering vehicle-miles-traveled-based user fees as one option.  The Government Accountability Office released the report Review of the Highway Trust Fund: The Viability of Mileage Fees for Certain Vehicles (PDF), which determines that the fees can lead to more efficient roadway use, but describes challenges such as privacy concern and the difficulty of developing a cost estimate.  The report also examines whether mileage fee rates are necessary to replace and supplement current Highway Trust Fund revenues.  RAND Corporation has released Mileage-Based User Fees for Transportation Funding: A Primer for State and Local Decision Makers, which presents promising and innovative mileage fee system designs and transition strategies for states or localities considering or refining mileage fees.  The report also summarizes the activities of states that have commissioned studies or piloted VMT-based fees, including Minnesota’s, which includes a pilot of volunteers in two counties that contain rural, suburban, and urban areas.