Public transportation, along with most government programs face a cyclical problem. Underfunded programs function poorly because they are under funded. The public sees that programs are poorly functioning and perceive them as being a waste. They then lower their funding to prevent "waste" which causes the program to function even worse and continuing the cycle. Transportation is always an easy target for policymakers trying to save money because the perception is always that it works terribly and wastes lots of money.
In reality, public transportation saves billions of dollars for Americans. If people had better access to it, it would save trillions, and would make everyone much more productive. People may have to spend a few dollars more in their taxes for public transportation, but those who choose to use it save thousands per year (net).
Note: I take the bus everyday to work (in one of the most dangerous cities in America) instead of my car and have decided to get rid of my car.