The Highway Trust Fund which pays for the IHS is funded by its users through taxation on gasoline and diesel. I wouldn't call that a weird parallel construction.
Fuel taxes only cover half the cost of roads. https://taxfoundation.org/data/all/state/gasoline-taxes-and-...
loading story #50249297
The result of this system is that if you happen to use less gas for any reason (certainly relevant in this day and age), your transit is essentially subsidized! That's the weird bit in my mind.
loading story #50247499