While freight transportation in Australia is known for its impressive "road trains"---one semi-truck-pulled train towed 104 trailers at once in 2006---regulations in the United States do not allow for more than three commercial trailers without special dispensations for unusual loads; two are standard. Recreational travellers are restricted to two trailers in many states; these rigs typically consist of a travel trailer followed by a boat or toy hauler trailer. The U.S Department of Transportation's (USDOT) Federal Motor Carrier Safety Administration (FMCSA) administers semi-truck and trailer regulations on the federally-designated national highway network, and the National Highway Traffic Safety Administration (NHTSA) legislates rules for personal and recreational trailer towing.
Semi-trucks pulling double or triple trailers are legal on the USDOT designated national highway network based on load weight restrictions. The laws of the NHTSA also establish some provisions each state must follow in regard to these trucks on secondary state highways; the rules essentially keep states from prohibiting truck and trailer rigs on secondary roads so long as they are within certain length and weight parameters. Drivers towing trailers must meet all the qualifications required by the FMCSA which include, but are not limited to being: 21 years or older, English-language capable, trained and licensed and not disqualified due to criminal behaviour while hauling or unsatisfactory safety or accident record.
In 2009, "Automobile Magazine" published a map of states that do and do not allow double trailer towing; when published, the only states that did not allow double towing were: Washington, Oregon, Maine, Vermont, New Hampshire, Massachusetts, Rhode Island, New York, Connecticut, West Virginia, Virginia, North Carolina, South Carolina, Georgia, Florida, Alabama and Hawaii. If you plan to tow a travel trailer with a boat behind it, for example, check the laws for each state you will travel through before you go; they may disallow double trailer travel entirely, or may allow it with restrictions on overall length of combined tow vehicle and trailers. The NHTSA also advises you to thoroughly familiarise yourself with the towing capacity first of the tow vehicle, with an eye to the combined load of both trailers, then to the capacity of the first trailer to maintain a following trailer safely. Although special permits may be required, you do not need a special driver's license to tow double trailers.
From time-to-time the FMCSA works with states to develop a new or temporary highway system in the network; this might occur in order to facilitate bringing oversized equipment into a mining or drilling facility. Most of the time, however, state's rules apply. If you drive semis or personal vehicles, you may be able to obtain special permits through each state's department of transportation (DOT) to tow double trailers, or oversized double trailers if you can demonstrate the need for such permits. For commercial drivers, it may be easier to use a multi-state private permitting service; but both tractor-trailer drivers and private individuals should be able to obtain necessary permits from the individual states' DOTs.