Isn't there something artificial about only using gas taxes for road maintenance? What other product do we tax that way?
We have special gas taxes simply because they are, or at least were, a politically palatable way of funding roads - a use tax, and extension of the idea of paying tolls for ferries, bridges and roads.
A lot of states have a DOT - that's T as in Transportation, not just highways. In Washington, DOT runs the ferries, and a network of small airports. It's also involved with some regional rail projects, just a NW corridor passenger line.