I don't know if it's a local thing, or all of SoCal or all of the US, but i keep seeing cars with these small (maybe 2 inches by 2 inches? or 4 x 4? i'm bad at judging such things) squares of paper taped to their rear window. They have white border and then a kind of faded redish sqaure center with white numbers in the center of that. When i first started seeing them i think they were in the 7ish range, but they've been climing since then. I started seeing 9s after awhile, and now they're into the double digits with 10s and 11s. I saw two of those yesterday, and then another 11 today, and then saw someone going by who actually had a 1.
I haven't noticed any obvious similarities about the cars that have them, and i can't imagine any usefull purpose they could serve, so what the hell are they for?