If fuel efficiency becomes a driving force for American autos, we will no longer be able to pay for road construction/maintenance without a shift in the current gas tax structure.
How, you ask? As it stands, the tax is based on X cents per gallon, with inefficient vehicles (SUVs, trucks) shouldering the lion's share of the gas tax fund (though, by no means, "paying their way"). If we all switch to lighter, fuel-efficient cars and rail, the interstate system would have to switch appropriation measures to a % of $/gal basis, because the income the current system produces would drastically decline every year, leaving highway lobbies in the soup line

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Will PO mark the end of the interstate system as we know it?
Will it end up being perceived as a subsidy, like transit currently is?