Vehicle Combinations. Generally, it is unlawful to operate on public highways any combination of vehicles consisting of more than two vehicles. However, the addition of another axle to the tractor of a truck tractor-semitrailer or pole trailer combination that supports a proportional share of the load of the semitrailer or pole trailer is not deemed a separate vehicle and is part of the truck tractor. Generally, the maximum combined length allowed for a combination consisting of a tractor and two trailers is 61 feet.
 
Intermodal Surface Transportation Efficiency Act. The stated objective of the federal Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) is for states to develop and operate an enforcement process that identifies vehicles of excessive size and weight, and provides a systematic approach to eliminate violations and improve highway conditions. ISTEA, in effect, imposed a freeze on states to restrict the operation of longer combination vehicles (LCV) on the interstate system to the type of vehicles in use on or before June 1, 1991. Under ISTEA, an LCV is a combination of a tractor and two or more trailing units weighing more than 80,000 pounds that operates on the interstate. Because of the freeze, states that did not allow LCV operations before June 1, 1991, are precluded from allowing them or from lifting restrictions that governed LCV operations as of that date.

The Washington State Department of Transportation (WSDOT) must allow combinations of vehicles consisting of a truck tractor and three trailing units to operate on designated highways, but only if the federal government approves a variance to the existing freeze of state law imposed by ISTEA. WSDOT may also specify by rule other operating conditions to ensure a safe and efficient highway system. WSDOT must collect certain data regarding the allowance of such vehicle combinations on public highways, and submit an annual status and performance report on the implementation of the variance.