Road damage scales with axle weight to the fourth power (w^4).
So if a 500lbs motorcycle is the unit of damage, a 4,000 lbs sedan does 4,096 units of damage (4096x as much), a 6,000 lbs SUV/pickup does 20,736x, a 14,000 lbs delivery van does 614,000x, and the 80,000 lbs US federal limit 5 axle semi-trailer does 90,800,000x the damage of the motorcycle.
So probably shipping stuff in trucks doesn't cost enough, and SUVs/pickups should cost 5x for road tax what sedans do.