When I was in the States once, after watching Mexicans floating across the Rio Grand river thought we would take a look. Halfway across bridge decided it was a bad idea as car would not be insured so decided to turn around, trying to get past American border control was a long experience, he was convinced we had picked up something elicit. Strange thing about the bridge was that there was a metal strip half way across marking the border, the American side was perfect tarmac, the Mexican side was not much better than gravel.