"Federal surface transportation programs are currently funded primarily through taxes on motor fuels that are deposited in the highway trust fund. Although there has been some modification to the tax system, the tax rates, which are fixed in terms of cents per gallon, have not been increased at the federal level since 1993. Prior to the recession that began in 2007, annual increases in driving, with a concomitant increase in fuel use, were sufficient to keep revenues rising steadily. This is no longer the case. Future increases in fuel economy standards are expected to suppress motor fuel consumption in the years ahead even if annual increases in vehicle mileage resume."
CRS Report for Congress, R42877
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html