[ndsipa_pomu]

I suspect the number of axles and wheels is to generally improve the strength/weight ratio rather than to minimise road surface damage.

[coryrc]

Nope, the requirement comes from government standards intended to reduce road damage and specifies maximum ground pressure.

[ndsipa_pomu]

Okay, so there is a maximum. Charging proportionate to the road damage costs would still change the cost benefits of using single large vehicles vs multiple smaller vehicles, or possibly lots more axles.