<Li> 40 ft (12 m) and 20 ft (6.1 m) trailer Rocky Mountain Doubles, 134,000 lb (61,000 kg) GVWR </Li> <Li> Double 20 ft (6.1 m) trailers . </Li> <P> The US federal government, which only regulates the Interstate Highway System, does not set maximum length requirements (except on auto and boat transporters), only minimums . Tractors can pull two or three trailers if the combination is legal in that state . Weight maximums are 20,000 lb (9,100 kg) on a single axle, 34,000 lb (15,000 kg) on a tandem, and 80,000 lb (36,000 kg) total for any vehicle or combination . There is a maximum width of 8.5 ft (2.6 m) and no maximum height . </P> <P> Roads other than the Interstates are regulated by the individual states, and laws vary widely . Maximum weight varies between 80,000 lb (36,000 kg) to 171,000 lb (78,000 kg), depending on the combination . Most states restrict operation of larger tandem trailer setups such as triple units, turnpike doubles and Rocky - Mountain doubles . Reasons for limiting the legal trailer configurations include both safety concerns and the impracticality of designing and constructing roads that can accommodate the larger wheelbase of these vehicles and the larger minimum turning radii associated with them . In general, these configurations are restricted to the Interstates . Except for these units, double setups are not restricted to certain roads any more than a single setup . They are also not restricted by weather conditions or "difficulty of operation". The Canadian province of Ontario, however, does have weather - related operating restrictions for larger tandem trailer setups . </P>

What is the width of a tractor trailer truck