Truck and trailer rules in Washington: brakes, size limits and intrastate carrier rules

This page reports what the official source says. It is not legal advice: ask the office named here. The sources are three sections of the Revised Code of Washington (RCW) and the Washington State Patrol's rule on transportation requirements, WAC 446-65-010. Federal background and the other places are on the parent page, trucks and trailers for tree work.

At a glance

Item	What the sources say
Agencies	Washington State Patrol for the carrier rules; Department of Transportation for size devices
Trailer brake weight	Brakes on all wheels, except a trailer of 3,000 pounds gross or less meeting two conditions
Breakaway	Brakes that apply automatically and stay applied at least 15 minutes after breakaway, for trailers over 3,000 pounds gross
Width	8.5 feet, with mirror and safety device exclusions
Length	40 feet single vehicle; truck and trailer combination 75 feet
Carrier number	Not published on the pages read

Trailer brakes and breakaway

RCW 46.37.340 says every motor vehicle, trailer, semitrailer and pole trailer, and any combination of them, on a highway in the state must have brakes that meet the chapter. Brakes must act on all wheels, with listed exceptions. The first is a trailer, cargo extension, semitrailer or pole trailer of a gross weight not exceeding three thousand pounds, provided two conditions hold: the total weight on and including the wheels of the trailer or trailers does not exceed forty percent of the gross weight of the towing vehicle when connected, and the combination can meet the stopping performance in RCW 46.37.351. Older trailers made before July 1, 1965 are excused when the weight on the wheels does not exceed two thousand pounds, and a towed vehicle in a driveaway or towaway operation is excused if the combination meets the performance rule.

The same section covers breakaway. Every trailer, semitrailer and pole trailer with air or vacuum brakes, and every one with a gross weight over three thousand pounds made or assembled after January 1, 1964, must have brakes on all wheels that apply automatically and promptly upon breakaway from the towing vehicle and stay applied for at least fifteen minutes. A motor vehicle made after that date and used to tow a braked trailer must be able to be stopped by its service brakes if the trailer breaks away. The section does not use the word chain. See RCW 46.37.340 on braking equipment.

Width and length

RCW 46.44.010 says the total outside width of any vehicle or load must not exceed eight and one-half feet. An externally mounted rear vision mirror may extend past the width limit to give the driver a view to the rear. Devices that provide added safety or energy conservation and are not designed or used to carry cargo are excluded from the calculation, and the Department of Transportation identifies them in rules. See RCW 46.44.010 on outside width.

RCW 46.44.030 makes it unlawful to operate a vehicle with an overall length over 40 feet, with or without load, with exceptions that include buses, fifth-wheel travel trailers and motor homes up to 46 feet. For a tractor and semitrailer, a semitrailer longer than 53 feet is unlawful, and a tractor with two trailers may not exceed 61 feet of combined trailer length. A combination of a truck and trailer, or a log truck and stinger-steered pole trailer, may not exceed 75 feet overall, with or without load. Vehicles carrying poles, pipe, machinery or other structural objects that cannot be dismembered, operated by a public utility for emergency repair of public service facilities, are outside the length limits, with clearance lamps and marker lamps required at night. The section was last amended by chapter 65 of the 2026 laws. See RCW 46.44.030 on maximum lengths.

Federal carrier rules inside the state

WAC 446-65-010, filed May 27, 2026 and effective June 27, 2026, says the State Patrol adopts listed parts of Title 49 of the Code of Federal Regulations, as they existed on April 25, 2025, for motor carriers used in intrastate or interstate commerce. The list includes Part 383 on the commercial driver's licence program, Part 390 general rules, Part 391 driver qualification, Part 392 driving, Part 393 parts and accessories, Part 395 hours of service and Part 396 inspection, repair and maintenance. Two intrastate carve-outs are written into the rule. Subparts D and E of Part 391, on tests and physical examinations, do not apply to carriers running vehicles with a gross vehicle weight rating between 10,001 and 26,000 pounds in intrastate work without hazardous materials needing placards, and the federal minimum age rule in section 391.11(b)(1) does not apply to drivers 18 through 20 years old operating solely in intrastate commerce under the same conditions. For hours of service, a company with drivers in dump truck operations or hauling logs from the point of production in intrastate commerce follows a separate set of limits within a 100 air-mile radius, with 12 hours of driving after 10 hours off duty and time records kept for 12 months. See WAC 446-65-010 on transportation requirements.

The pages read do not say whether a tree service is a dump truck operation, and they do not name a state carrier number. They also give no oversize permit rules, which sit elsewhere in RCW 46.44.