Unless you're mentally ill, making a statement, or are so desperate b/c you've already 'reached a point of no return', there should be no excuse for doing something like this in broad daylight right out in the open. I mean - have they heard of "woods" or at the very least, "deep bushes"?

Also, in most populated places in the US you're never more than 10 minutes away from a fast food joint, convenience store, or gas station. And as long as you're not a complete jerk you can usually wiggle around the 'restrooms for customers only' thing if they try to enforce it in the first place (In my experience, just walk up to the counter and politely ask for the restroom key.) Or just tell them you've got to go so bad and promise you'll buy something after you're done, then just leave... Who are they gonna call on you?

IBD really sucks sometimes... One time I was stuck in traffic during rush hour on the Mass Pike, and had pull over and make a break for the tree-line. Not proud of it, but it happened. It's one of the reasons that I strongly feel that freely available public restrooms should be a human right.