For those wondering about the source of the "4th power of weight per axle", I found this after some short, but intense, Googling:

http://co-asphalt.com/wp-content/uploads/2015/03/TrashTruckD...

Reference 1 lead me to this: (Page I-11) https://habib00ugm.files.wordpress.com/2010/05/aashto1993.pd...

From that: "The load equivalency factor increases approximately as a function of the ratio of any given axle load to the standard 18 kip single axle load raised to the fourth power."

That is also cited here, on page 14: https://wisconsindot.gov/Documents/dmv/agri-eq-veh/ag-study/...