If you are interested (why?) you can find a full listing of plate codes by county here.
1J stands for Jefferson county, which covers a few small towns in Eastern Idaho. I don't know what it is about residents of this county, but all the crazy drivers on the roads seem to have '1J' plates.
Maybe it's due to the fact there aren't any major cities in Jefferson county? It, like many parts of Idaho, is full of small towns and a lot of open farm roads. Not a lot of traffic, and you can usually drive like a kook without a lot of consequence.
If you want to go to the movies, you get to brave the 1J crowd.
If you are going to the mall and the roads are icy, beware the 1J-ers... they slide. A lot.
Even when I'm 250+ miles west, and driving around in Boise, a stray 1J-er will appear on the road, changing lanes aggressively without signaling, and often chatting/texting on their phone.
(which is legal in the state of Idaho...)
1J-ers are usually overly-aggressive or a little unaware of their surroundings. If you spot a 1J vehicle, be sure to give a lot of extra room, and keep your distance. Eye contact only enrages them, so make sure to avoid provocation of any kind.
Are there any crazies where you live? How do they make themselves known? Please share by commenting below, and make sure to beware any Idaho plate with a 1J moniker....