There are several well considered comments in this thread. The federal gasoline tax has gone from 3 cents/gallon in 1956 to 18.4 cents/gallon in 1993, and it hasn't changed since. That's 18 years without an increase in user fee revenues. Hybrid/electric vehicles are getting a serious subsidy, paid for by you and I, the gas/diesel users of this nation.
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Can you guess what construction costs have done over that same period?
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The Highway Trust Fund paid for federal US and Interstate Highway initiatives until 1983, when the populous east/west coast Democrats forced Reagan to accept 1 cent out of 5 cents revenue increase to go specifically to a new Mass Transit Account in the HTF. Gasoline provides 64% of HTF revenues, followed by Diesel at 24%, Heavy truck/trailer sales ~8%, and the balance from annual truck registration and truck tire taxes.
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If you look hard, you will see there is NO revenue into the HTF from any mode of transit - only a great sucking sound of our user fees being siphoned off for bus, subway, light rail, bike, etc routes that have NOTHING to do with Interstate Commerce. But, these non-revenue modes are getting more of our motor fuel taxes every year - leaving less for highway maintenance or expansion.
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The average total annual revenues for the HTF over the past several years is around $36B. The Mass Transit Account is pulling right at $5B per year out of that total, plus transit-friendly states are able to "flex" another $1B of their federal highway allocations to transit, while road-friendly states can't reverse-flex. About $3-4B/year of transit is funded by the decades-old General Revenue (income taxes) - for a total of around $10B per year for transit projects. Highways, despite collecting $36B/yr, are only netting around $30B in HTF money.
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Here's where it gets weird - the current transportation bill - SAFETEA-LU (now 2 years past its end) has been funding highways at around $40B per year - because the politicians do generally understand the necessity to support highway infrastructure for interstate commerce. However, they are ALL loathe to increase revenues to match expenditures. Furthermore, they have no qualms about taking yours and my national road user fees and giving them to local transit systems. The yellow bars from '07-'10 show repayment of HTF monies that were pilfered over the years for non-transportation purposes. These have pretty much been fully repaid out of G.R. (thru deficit spending).
We are now looking at the strong likelihood of a fully revenue-aligned transportation program in the next federal multi-year authorization (name your '-TEA') That means an across-the-board cut of approximately 30% from current levels. If you think your roads and bridges are in poor shape now - just wait...