A shift to alternative fuels and more fuel efficient vehicles might make good sense for the environment, but an unintended victim could be the condition of the country's roads and bridges.

Federal and state gas taxes charged at the pump pay for local, state and federal roads and bridges. Motorists in Michigan pay 18.4 cents a gallon in federal tax that goes to the Federal Highway Trust Fund and 19 cents a gallon in state tax that goes to the Michigan Transportation Fund.

Because the taxes are the same whether gas is cheap or expensive at the pump, tax revenues can decline if pump prices climb high enough where people change their habits, drive less and buy less gas.