The real problem with declining transportation revenues is that people are driving as many or more miles, but their vehicles get much better gas mileage than they used to. Back when the gas tax was relevant (hasn't been raised, even for inflation, since the early 1990s), the average car+light truck typically got 18-20 mpg, with really thrifty little cars getting perhaps 30 on the highway. Not much gas mileage range, so everybody payed an amount more or less proportional to how much they drove. Because of CAFE changes and reaction to higher gas prices (gas now is relatively cheap at near $2/gal even in N.Calif, if you do a little shopping, but that can't last), the typical vehicle (even a light truck) now averages mid-20s, and smaller/hybrids get much better than that (40s on the highway are fairly common, 40-50 in town happens with hybrids). So we're driving more but using less gas doing it.

The gas tax is primarily per-gallon, not per-dollar and certainly not per-mile. Even if the gas tax had followed inflation, we would be in trouble due to that, since maintenance costs are pretty much proportional to miles driven and vehicle weight (everything's gotten heavier, too, with increased safety standards). Since it hasn't, we get a double whammy. Then BART project costs do their normal super-escalation (well beyond the inflation rate, even for general construction which is higher than the CPI). And we get a Congress that wants to shift the remaining funding back to roads and let transit go hang. Any doubts about why a BART project is hitting funding walls?