Tandem semitrailers are big rig trucks pulling two trailers, one behind the other. They are officially comprised of a tractor, semitrailer and trailer and have specific limitations set by the U.S. Department of Transportation, Federal Motor Carrier Safety Administration. They are allowed to carry more weight per tandem axle than traditional semitrailers on single axles, but are still restricted by the overall regulations.
The gross weight on any tandem axle should not exceed 34,000 pounds as long as the total gross weight of the entire vehicle is not more than 80,000 pounds. In the event the tandem trailer uses idle reduction technology or auxiliary power to reduce emissions and save fuel, an additional 400 pounds is allowed.
Tandem trailers should not exceed 65 feet in length and individual states may not limit the length of any semitrailer to less than 28 feet. Tandem trailers are allowed on all national roadways, provided construction does not inhibit their use.
Assuming the tandem trailer has a gross vehicle weight rating that is more than 10,000 pounds, one wheel on any tandem axle may lock during a stop when the vehicle is traveling at a speed of 20 miles per hour or less. During this process the vehicle should remain aligned on the road and no part of the vehicle should ever leave the roadway.
Each state sets its own truck regulations for its roadways, including those for tandem trailers. Tandem trailer drivers wishing to use roadways not part of the national network must obtain a permit from the individual state's department of transportation. A list of size regulations by state is available at the U.S. Department of Transportation's website.