I came close a few months ago.
Driving along the trans-canada highway near Banff, I had the urge. Of course, I was in the 60km stretch where there is basically no turnoffs at all, nowhere to pull off the highway. So I'm driving 160km/hour trying to get to the next rest stop, all the while pretty much standing up in my car so I can 'clench' hard enough.
Finally get to the rest stop, which luckily is a bit off the highway near a river, so it's very much out of sight of the highway. Pull in, bolt out my car into the woods, do my stuff. Have to use my boxers to wipe :\
As I'm coming back to my car, 3 more vehicles pull in, all to walk their pet dogs. I left rather fast.
The kicker was that the next day I found a roll of toilet paper under the drivers seat, which I had put there months earlier for situations like that. In the heat of the moment though, I had forgotten.
