Last year, I was pretty bored being stuck in rush hour traffic so I decided to make my commute a little more productive (or useless, depending on your point of view), by making a note of the location of different license plates. The following figure is a compilation of a year’s worth of data:
There are several caveats about this geographic heat map. Since the observations were done primarily in California, of course the top number of license plates will be California. I also recorded plates from Canada (a couple) and Mexico (mostly from Baja California and about as many as Arizona plates). I actually saw zero North Dakota plates during the observation period, but about one week after the observation period ended, I saw two separate vehicles with North Dakota plates.
I suspect the map would look vastly different if I lived in a different Californian city (particularly one where there is almost no tourism), let alone in a different state.
The most frustrating aspect about the data collection was that sometimes it was impossible to tell where the plate was from because of the way it was mounted on a vehicle. Sometimes the license plate holders would cover all of the license plate except for the alphanumeric code. Occasionally, I could guess fairly confidently what the plate was if the font/color/design matched the default plate of that state, but if someone had a vanity plate, I had no clue. In the cases where I was unsure, I did not record them.