Source: Boston Globe
quote: Based on the findings of the American Association of State Highway Officials (AASHO) road test, damage caused by heavy trucks was long thought to increase with approximately the fourth power of the axle load. This means that one axle of 10 tons on a heavy truck was 160,000 times more damaging to a road surface than an axle of 0.5 tons (car scale). In recent years, however, it was determined that the relationship between axle weights and pavement damage is complex and varies based on numerous variables, including environmental factors, type of terrain and roadway design. The National Pavement Cost Model (NAPCOM), which is the pavement model currently used by FHWA, estimates that for some types of pavement deterioration, doubling the axle load causes 15 to 20 times as much damage; for other types of deterioration, doubling the load only doubles the damage. The U.S. Department of Transportation in its most recent Highway Cost Allocation Study estimated that light single-unit trucks, operating at less than 25,000 pounds, pay 150 percent of their road costs while the heaviest tractor-trailer combination trucks, weighing over 100,000 pounds, pay only 50 percent of their road costs.