Paying for roads in the United States was once straightforward. Petrol (gasoline) taxes, easy to administer and collect, were directed into the federal Highway Trust Fund. Those revenues were in turn distributed to the states for maintenance and investment. State petrol taxes provided further funds. And because petrol consumption was a rough proxy for road use, the system satisfied the principle that consumers should pay for a product in proportion to their use of it. That old model no longer works. First, the federal petrol tax has failed to keep pace with prices: it was last raised, to 18.4 cents per gallon (3.8 litres), 20 years ago. Most states also do not index their petrol taxes. Second, petrol taxes no longer serve as an adequate stand-in for road use. Cars are becoming increasingly fuel-efficient (see chart 1): by 2025 new vehicles must, by law, run at an average 54.5 miles per gallon (mpg).
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