Yeah, in Mexico all traffic offenses are both civil and criminal offenses. Fuji's right, if you can't prove either MEXICAN insurance (no American company is valid over there) which you're supposed to buy just over the border, or wherever the rented car is from (and costs out the ass) or a wallet full of dough, you can expect to spend at least a couple nights in jail while they do the "paperwork" (make you sweat).