for the rear trailer, and a tongue with a ring-hitch coupling for the forward trailer. Individual states may further allow longer vehicles, known as "longer combination vehicles" (or LCVs), and may allow them to operate on roads other than Interstates. Long combination vehicle types include: Doubles (officially "STAA doubles", known colloquially as "a set of joints"): Two trailers. B-Doubles: Twin trailers in B-double configuration (very common in Canada but rarely used in the United States). Triples: Three trailers. Turnpike Doubles: Two - trailers. Rocky Mountain Doubles: One trailer (though usually no more than ) and one trailer (known as a "pup"). In Canada, a Turnpike Double is two trailers, and a Rocky Mountain Double is a trailer with a "pup". 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 on a single axle, on a tandem, and total for any vehicle or combination. There is a maximum width of and no maximum height. Roads other than Interstates are regulated by individual states, and laws vary widely. Maximum weight varies between to , 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 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. Oceania Australia Australian road transport has a reputation for using very large trucks and road trains. This is reflected in the most popular configurations of trucks generally having dual drive axles and three axles on the trailers, with four tires on each axle. This means that Australian single semi-trailer trucks will usually have 22 tires, which is generally more than their counterparts in other countries. Super single tires are sometimes used on tri-axle trailers. The suspension is designed with travel