According to Washington Administrative Code 468-38-100(8):

In addition to being in safe and reliable operating condition, the vehicle shall:

(a) Be either a single unit passenger car, including passenger van, or a two-axle truck, including a nonplacarded service truck;

(b) Not exceed a maximum gross vehicle weight or gross weight rating of sixteen thousand pounds;

(c) Have a body width of at least sixty inches but no greater than one hundred two inches;

(d) Not exceed the legal limits of size and weight, as defined in chapter 46.44 RCW;

(e) Be equipped with outside rear-view mirrors, located on each side of the vehicle; and

(f) Not tow a trailer while escorting.