This is not apples-to-apples, because buses don't drive on all of the roads, whereas cars drive on (presumably) any road. The total mileage of roads used for bus routes is a small fraction of all road mileage, even where bus systems are extensive. So more roads must be maintained for cars. Road damage is also affected by vehicle speed, acceleration, braking and so forth; buses drive regular routes at regulated speeds and do not brake very hard.