If everyone who normally takes the car would switch to taking the bus, all of a sudden you’d only need one lane in places where you previously needed two or three, because cars are very space-inefficient, so that makes a big difference.

Also, it’s not quite true that they’d require the same amount of roads. I don’t know about where you’re from, but where I live buses use about a quarter of the roads and you can still easily get anywhere by bus.

Additionally, salt isn’t used for rail vehicles at all.