From empirical studies, damage to the road is proportional to the fourth power of axle weight. A bike with rider may weigh 200 pounds, where a passenger car weighs around 4000 pounds. That 20x difference in weight results in a 80,000x difference in damage to the road.
(That’s not even getting into semi trucks, which are around 40 tons fully loaded. Split along 5 axles rather than 2, that’s 9x the axle load of a passenger car, leading to 6,500x the damage to the road relative to a passenger car, or 520 million times that of a bike.)
