Car-related taxes (vehicle sales, gas tax, yearly registration fees, in some cases tolls) have historically covered the majority of roadway infrastructure costs. I don't think free buses are going to be able to maintain the roadways.

A common misconception; usage fees only covered about 50% of highway-related expenses a few years ago. Feel free to find the latest numbers. This is less than the fare box recovery ratio of many transit systems, though not all.

And that, of course, does not include all the unpriced externalities of roads. For example, if you value a life at $1M, then the 40,000 people killed by drivers each year cost us collectively another $40B.