Apparently in California there are regulations about what you can put in your car's windows. The law is Vehicle Code Section 26708 [1], which states that "No person shall drive any motor vehicle with any object or material placed, displayed, installed, affixed, or applied upon the windshield or side or rear windows."

Now before you go and throw out your toll roads transponder, your rear window wiper motor, and your sunvisor, there are exceptions to this rule. (Including the three I just mentioned). They are listed in 26708(a)(3b) of the above code.

The one I am most interested in is (3b)(3) which states that

Signs, stickers, or other materials which are displayed in a7-inch square in the lower corner of the windshield farthest removedfrom the driver, signs, stickers, or other materials which aredisplayed in a 7-inch square in the lower corner of the rear window farthest removed from the driver, or signs, stickers, or othermaterials which are displayed in a 5-inch square in the lower cornerof the windshield nearest the driver.

This would seem to affect a number of drivers on the road, like the truck I saw the other day with one of those urinating Calvin stickers [2] and that fluffy Garfield [3] suctioned to the center of the back window.

I first discovered this law referenced by Garmin [4], warning that Californians are prevented from "using suction mounts on their windshields." I was unable to find suction cups specifically mentioned in the Vehicle Code, and it appears to me that a GPS unit affixed by suction cups to the bottom left corner of the windshield would be permissible if it was within a 5-inch square. I am not a lawyer however.