I see a lot of cars and trucks in Washington that don't have license plates installed on the front. When we bought our family car a year or so ago, there was a length of time when it also didn't have its front plate installed. The car was most likely purchased by the dealer out of state, where two license plates aren't required. Needless to say, I installed the hardware, put the plate on, and never thought about it again.
So, what does Washington state law say about having two license plates installed?
RCW 46.16A.200 (the law on license plates) is actually quite long, making note of special exceptions, and elaborating on using stolen plates; let's just say it's pretty thorough. But let's get straight to what you came for. In Washington state, license plates must be:
"Attached conspicuously at the front and rear of each vehicle if two license plates have been issued;"
If you're wondering what "if two license plates have been issued" means, it's actually pretty simple. Not all vehicles will have two plates. Common examples of this are motorcycles and trailers. Washington state law also grants state troopers the ability to use their discretion in certain cases, such as "if the body construction of the vehicle makes compliance [...] impossible."
There you have it; a pretty straightforward law that does allow for some wiggle room in special instances.
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