Just like the schools. The highways are maintained by either incompetent/political/time clock punching state bureaucracies. The repairs are done by overpaid, unproductive union labor with that great union quality. The contractors are usually hooked up firms that get the jobs for political payoffs. The highways themselves suffer from the classic economics case of the tragedy of the commons.
Like everything else, they are well funded. However there is never enough to fund beyond the appetites of players involved. Politicians, unions, contractors.
My solution?
Get rid of the federal gas taxes, and sell, permanently, the highways. All of them.
They are too important to be in the hands of incompetent, high cost, poor quality government.