Thank God for the internet. I'll try to look up some facts in my spare time over the next couple of weeks. feel free to reply me or freepmail if you have anything hard on this.
Think of 3 concentric circles of highway funding.
The first concentric circle is the gas tax. Here in Washington, the 18th Amendment to the state constitution requires that it be spent on roads. In 1967, the state supreme court added ferries to the mix because state ferries carry numbered state highways over large bodies of water, like Puget Sound. (It's like a moving bridge.) But that's it. Not a penny for bike paths, the state rail program, port improvements or transit support. It's 100% for highways and ferries.
The second concentric circle consists of other car-related taxes, like the hated Motor Vehicle Excise Tax (MVET). These can be used for anything in the Washington State Department of Transportation's budget, including the forbidden items I mentioned above. But about 95% goes for highways.
The third concentric circle is the general fund, which in Washington consists of sales taxes, business & occupation taxes, and the state cut of property taxes. This may be used for anything anywhere in the state's overall budget, but about 90% of the transportation component goes to highways.
When the voters cut the hated MVET down to a flat $30 per vehicle, the second concentric circle contracted. To continue building highways -- we're 20 years behind schedule -- we either need to increase the first (gas tax) or third (general fund) concentric circles. Realizing that the "road gang" was going to attempt a grab of the general fund, the teacher's union successfully passed an initiative via the voters that permits them to insert a vacuum hose of a fixed diameter into the general fund for perpetuity. So now we're fighting over the first concentric circle.
In some states and at the federal level, the first concentric circle is not highways-only, but more generally related to transportation needs. But the funding scheme is pretty much the same everywhere.