Sure, but they do contribute to the cost of maintaining roads. We need to figure out roughly what percentage of road maintenance costs come from vehicles and ch

Sure, but they do contribute to the cost of maintaining roads. We need to figure out roughly what percentage of road maintenance costs come from vehicles and charge individual vehicles based on their rough contribution to maintenance costs. It's not a terribly hard calculation, especially since you can directly compare with roads that are infrequently used by trucks.