While it may be true that the increases in MPG in the cars that use the roads has resulted in an effective decrease in the gasoline-based tax revenue on a per car basis, part of the problem is that the money generated from gasoline taxes is often robbed for uses other than roads and bridges. This was not always the case like it is today. Like Social Security, the revenues from the fuel taxes are collected under the guise that they will be reserved for a certain purpose but in fact, they go into the general fund and at that point, they seem to be considered fair game.
For this reason, while the streams of revenue for roads has continued, the money allotted to the improvement and manintenance of the highway infrastructure has lagged, especially with the economy being in such a poor state and the fact that many other government programs were expanded when times are good and there seems to be a reluctance to scale back on such things when times are tight while they seem to be fine with cutting funding for infrastructure during these lean times.
Also, it used to be that the funding of toll roads was separate from funding for non-toll roads in that toll roads had to be self sufficent in that they did not receive any tax revenue. However, that is no longer the case and they play so many shell games with the funding that it is difficult to say where the money comes from. It may be that for the reasons stated above, they do not want people to know where the money comes from or where it gets spent.
