Another question that comes up often in Michigan relates to our extra heavy trucks. While most states allow up to just 80,000 pound GVW trucks, Michigan allows up to 164,000 pound GVW trucks. Ontario has a similar system. Generally, the public perception is that these extra heavy trucks that Michigan allows are responsible for a large amount of the damage to Michigan roads. However, engineers generally believe that Michigan’s heavy trucks actually impose less damage than standard weight trucks because of the way loads are required to be spread over multiple axles. The heaviest Michigan trucks are required to have 11 axles, and these trucks are allowed to have a maximum weight per axle of 13,000 pounds, as compared to standard 5-axle trucks that can have up to 18,000 pounds per axle. Engineers generally believe that it is axle weight that damages roads, not overall gross weight.
During Michigan House of Representatives hearings on this issue in 1990, MDOT Director Jim Pitz indicated that engineering tests suggested that 13,000-pound axle loads would result in 62 percent less stress to the road than would be the case with 18,000-pound axle loads. During the same hearings MDOT officials testified that all state trunkline system bridges built after 1973, and all bridges reconstructed since that date, had been designed and built to withstand the full weight of 164,000-pound trucks. The additional cost to take these bridges up from 80,000-pound design specifications was 4 percent, or about $16,000 per bridge in 1990.
Nor are there a large number of 80,000 to 164,000-pound trucks on the road. While more recent data is not available, a 1998 MDOT report indicated that there were about 15,000 over-80,000-pound trucks registered in Michigan at that time, with less than 5 percent of the total truck traffic licensed to carry over 80,000 pounds. Also, even when licensed to carry heavier loads, these trucks often are carrying loads below what their license allows. Limiting trucks to 80,000 pounds would substantially increase the number of trucks on the road, with negative implications for safety, fuel consumption and air pollution. It also should be noted that the heavier weight limits in Michigan provide a significant advantage to Michigan manufacturers. In 1993, testimony by the Michigan Trucking Association estimated that an 80,000 pound limit would require an additional 21,500 trucks at an acquisition cost of $2.15 billion and annual operating costs of $0.77 billion. Of course this cost would be passed on to manufacturers and ultimately consumers.