The basic semitrailer limit is 53 feet. Yearly permits allowing the use of 56-foot chassis are available on an “automatic” basis, for the asking. Washington's basic box length is 61 feet, which allows the operation of two 28-foot trailers. A permit allowing a 68-foot box length is available across the counter for a fee of $100. Anything greater than 68 feet requires an irreducible load permit.
An increasingly common combination involves a straight truck and trailer using a long drawbar designed to increase the allowable GVW in accordance with Bridge Formula B to the maximum of 105,500 pounds. One example is the “4-4” combination, a four-axle straight truck with a single lift axle coupled to a four-axle trailer with two lift axles.
The Stinson trailer is a specialized unit designed to transfer containers between Seattle and Vancouver. The unit is comprised of a three-axle tractor, a single axle jeep, and a tandem axle semitrailer with a rear booster axle. This vehicle is designed to meet the Bridge Formula B requirements hauling fully loaded 40-foot containers within the 105,500-pound GVW limit. There are about 25 of these units in regular operation.
Tridem axles in six-axle tractor-semitrailer combinations are increasing in popularity because of carriers requirements to practice “water level loading” (that is, to load their vans at a more or less constant level without having to worry about axle weight distributions). Such loading might lead to a weight distribution such as 11,000 pounds on the steering axle, 27,000 pounds on the drive tandem axle, and 42,000 pounds on a wide-base tire tridem axle, for a total of 80,000 pounds.
There have been requests to permit Canadian trucks to operate at RTAC weights from the Port of Bellingham to deliver logs and timber to the Socco plant in Sumas without requiring down- loading.
The limits specified for Idaho by the ISTEA freeze are 105,500 pounds on Intersate highways and a box length of 95 feet for NN highways for double- and triple-trailer combinations. In general, permits are required to operate at greater than 80,000 pounds GVW on IS highways, but these are obtained across the counter for a $5 fee.
Seven-axle tractor-semitrailer and truck-trailer combinations are used in Idaho. The seven-axle tractor-semitrailer consists of a four-axle tractor with a (wide-base tire) lift axle between the steering axle and the drive tandem, and a tridem axle 53-foot semitrailer (using dual or wide-base tires). Typical maximum weights might be 12,000 pounds on the steering axle, 8,000 pounds on the lift axle, 34,000 pounds on the drive tandem axle, and 42,000 pounds on the tridem axle, for a total of 96,000 pounds GVW. Such units are used in chip haul and the trucking of hog fuel from forest to rail.