This piece lays out the argument, which says that the government's trend towards applying the gas tax to various transportation projects reduces its efficiency. By directing it solely at the interstate highway system, there would be no reason to raise the tax, according to the report.
"[A]sking federal highway users to pay substantially more in order to fund expanded programs for sidewalks, bikeways, recreational trails and more transit is unlikely to succeed, since the large majority of highway users do not use, and would not benefit from, these mostly localized urban projects. Principles of federalism suggest that these kinds of projects are more appropriately funded at state or local levels of government. But if Congress sees fit to continue them at the federal level, they should be supported by all taxpayers, as the kind of social infrastructure funded by federal agencies concerned with urban amenities (HUD) and outdoor recreation (Interior)."