The United States Department of Transportation (DOT) Federal Motor Carrier Safety Administration (FMCSA) division regulates the maximum weight of vehicles traveling on interstate highways. Guidelines for weight are defined in Part 658.17 of the FMCSA rules and regulations.
All commercial motor vehicles must adhere to gross vehicle weight limits.
The maximum gross vehicle weight on interstate highways is 80,000 pounds. If the bridge formula requires a weight lower than 80,000 pounds, that lower weight will prevail.
There are weight requirements for single and tandem axles.
No single axle can carry more than 20,000 pounds of weight. Tandem axles cannot exceed a weight of 34,000 pounds.
Bridge Gross Weight Formula
The bridge gross weight formula protects bridges from heavy vehicles.
The bridge gross weight formula decreases the gross weight for shorter vehicles because their weight is distributed over less area. Under this formula, no two consecutive axles can carry a weight greater than that determined by the following formula, even if the gross vehicle weight is less than 80,000 pounds: W=(ln/(n-1)+12n+36) [where: w=gross weight (to nearest 500 pounds); l = spacing between the outer axles (in feet); n=number of axles].
If the first and last axles of two consecutive sets of tandems are more than 36 feet apart, the bridge formula does not apply. Individual states can issue overweight permits on a case-by-case basis. States also issue their own weight limits for intrastate highways.