The fourth power law is only an approximation. If the road is designed for higher weight then the impact of larger loads is less. From https://en.wikipedia.org/wiki/Fourth_power_law

> A 2017 report commissioned by the New Zealand Transport Agency found a wide variation in the best-fitting exponents for a power law on 4T axle loads vs 6T axle loads, depending on the current condition and type of the roading. As a very rough summary of its highly detailed findings: A 9th-power law is most predictive when the road is barely able to withstand the 6T load; and the per-crossing damage is roughly linear to axle-weight when the pavement is able to withstand much higher loads than 6T per axle.

Highways (which this link focuses on) are designed for a heavier load than, say, residential streets.

A mid-size SUV is, what, 1 ton per axle? And a semi is max about 10 tons per axle (I don't know the average). And there are more SUVs on the highway than commercial trucks.

And in any case, there's already a Heavy Vehicle Use Tax which is meant to fund the additional maintenance demands caused by vehicles over 55,000 pounds.