Just to help people out, I checked for Washington state, and here's what it says:
Non-reflective tint is allowed on the top 6 inches of the windshield.
Front Side Windows - Must allow more than 35% of light in.
Back Side Windows - Must allow more than 35% of light in.
Rear Window - Must allow more than 35% of light in.
Front Side Windows - Must not be more than 35% reflective.
Back Side Windows - Must not be more than 35% reflective.
Restricted Colors - The tint color(s) of RED are not legal by state law.
Side Mirrors Dual side mirrors are required if back window is tinted.
Certificate Requirements - Manufacturers of film need to certify the film they sell in the state. Ask your dealer if they are using ceritified film.
Sticker Requirements - The sticker to identify legal tinting is required between the film & glass on the driver’s side window.
Medical Exemption - State allows medical exemptions for special tint. For more details about the specific terms of the exemption, consult your state law.

http://www.tintcenter.com/laws/WA/