Yes I have. Sometimes just because there were no rest stops available, and a couple of times because I was so desperate, it was either pee in car or pull over quickly and pee on the side of the road.
One of my memorable times was while I was out driving around the city, I miss gaged my bladder and became very desperate. I was on the freeway with my friend (before she knew I had this little fetish) I really needed to pee so I quickly pulled off in to the access road thinking that I would pull off into a side road and go. Nope, had to pull off to side of the road, quickly hopped out squatte