edit just to make it clear: This is not my opinion. This is the legal definition. The study includes citations as arrests. This is why they make sure to point out that they do not include minor traffic offenses.
I very strongly agree with Janice Rogers Brown's opinon here: http://www.volokh.com/posts/1125942214.shtml
edit: Hmmm after reading a few things, I now think it depends on the jurisdiction whether they define receiving a citation as a non-custodial arrest.
Depends. Just need probable cause. Motor vehicle exception.
>But they do have that right if they arrest you.
Depends. Search incident to arrest exception with automobile caveats. After Gant, if the arrestee no longer has access, or no reason to believe that evidence of the arrest offense will be found, not ok. (But still maybe ok to impound and inventory.)
Also with arrest, lower standard for if they think evidence will be found. It's the RAS-like "reasonable to believe" that evidence of the crime will be discovered.
I'm probably in the squeaky clean category. On a day of a meteor shower, I went with some friends to take pictures of the sky. A park ranger arrested us, for being in a national park after dark. After arresting us, he spent 5 mins talking on the radio with some other ranger, then spent 5 mins writing a citation. Paid it in the mail a few weeks later, never heard of it again.
It doesn't take much to be arrested. 40% is a bit high, but then again my group of friends at the time was Harvard/Princeton/Stanford/Berkeley students. The socioeconomic status of the population really doesn't matter.