Well, a quick search of the good ol' Washington Administrative Code reveals that sway bars are not required in Washington State for
FRONT SUSPENSION. They are howevor required for rear coil sprung vehicles.
So, if you are driving a vehicle with a rear coil spring suspension, a rear sway bar is required.
Otherwise, you have to meet the cone-swerve test requirements.
See
http://apps.leg.wa.gov/wac/default.aspx?cite=204-10-036
Which says:
WAC 204-10-036
Suspension.
A motor vehicle must be capable of stable, controlled operation while traversing a slalom-type path passing alternately to the left and right of at least four cones or markers arranged in a straight line and spaced sixty feet apart at a minimum speed of 25 mph.
Body lifts are permitted provided that they are manufactured by an aftermarket manufacturer, designed for the make and model vehicle on which they are installed, and installed according to the manufacturer's recommendations. Body lifts may not use more than a three-inch spacer and may not raise the body more than four inches above the frame when all components are installed.
A motor vehicle must:
(1) Have a minimum ground clearance to allow the vehicle to be in motion on its four rims on a flat surface with no other parts of the vehicle touching that surface and a maximum ground clearance determined based on the table contained in WAC 204-10-022 (6)(f) bumpers.
(2) Have spring mounts and shackles properly aligned and of sufficient strength so as to support the gross weight of the vehicle and provide free travel in an up and down movement under all conditions of operation.
(3) Incorporate antisway devices to control lateral movement in rear coil spring suspension systems.
(4) Have a suspension system that allows movement between the unsprung axles and wheels and the chassis body.
(5) Be equipped with a damping device at each wheel location. The dampening device must stop vertical body motion within two cycles when any corner of the vehicle is depressed and released.
(6) Be capable of providing a minimum relative motion of plus and minus two inches.
(7) Not use heating or welding for coil springs, leaf springs, or torsion bars.
(8) Not be constructed or loaded so that the weight on the wheels of any axle is less than thirty percent of the gross weight of the vehicle.
(9) Not raise or lower the height of a motor vehicle while the motor vehicle is traveling more than 15 mph on a public roadway with a posted speed limit of 25 mph or less. Except when lawfully participating in a parade permitted by local jurisdiction.
(10) At no time have any portion of any tire of such motor vehicle leave the surface of the roadway.
(11) Not have any portion of the vehicle or component of the hydraulic system used to raise or lower the vehicle cause or emit sparks.
Nothing in this section shall prohibit a county or city from enacting stricter regulations for aftermarket vehicle hydraulics on a public roadway.
