Looked into this mudflap issue a bit more for our build. Comments welcome, especially from any law experts.
It looks like:
A) Washington State would not require mud flaps if you have fender flares (eg Bushwackers)
Fender Flares & Anti-Spray Devices
Required - All vehicles, trailers, and semi-trailers must be equipped with fenders
, flaps, or
splash aprons adequate for minimizing the spray or splash of water and mud to the rear if the vehicle.
If I'm reading this correctly, Bushwacker-type fender flares would also satisfy the Utah mud flap law.
"41-6a-1633. Mudguards or flaps at rear wheels of trucks, trailers, truck tractors, or altered motor vehicles -- Exemptions.
(1) (a) Except as provided in Subsection (2), when operated on a highway, the following vehicles shall be equipped with wheel covers, mudguards, flaps, or splash aprons behind the rearmost wheels to prevent, as far as practicable, the wheels from throwing dirt, water, or other materials on other vehicles: ....
(2) Wheel covers, mudguards, flaps, or splash aprons are not required:
(a) if the motor vehicle, trailer, or semitrailer is designed and constructed so that the requirements of Subsection (1) are accomplished by means of fenders, body construction, or other means of enclosure
Note that this phrasing/subsections doesn't put any size requirements re the fenders.
C) Broader legal question:
Related to these sorts of laws about details of a vehicle build, what is required of visitors to a state?