oregon has similar laws for trailers. Washington basically requires everything to be licensed. I traveled with the oregon legal trailer (motorcycle hauler) to washington quite often. Finally got a oregon plate for it because for whatever reason it gave WA LEO's a supposed reason to stop me. Usually for no plate, even though I think they new oregon didn't require it. Then ask for registration, insurance, make sure lights worked, etc. etc. trying to find an excuse to write a ticket. Never got a ticket, but IMHO it gave them a fake excuse for a stop. Usually got pulled over with an LEO coming up behind me so they wouldn't know I was from oregon and hence completely legal in WA. But once stopped, all the reason to hassle me.