Ah, the memories...

My mom had a similar idea when my brother and I were both little kids. I was sitting in the front seat of the car, and my brother was in the back seat. Mom was driving. We're on the highway in traffic that was moving at barely a crawl through an extended construction zone. There were Jersey barriers on either side of the road, and no shoulders. My brother starts complaining that he has to go to the bathroom. My mom tells him there's nowhere to pull off, and he's just going to have to hold it until the next exit. My brother says that he needs to go NOW, so my mom hands him a paper cup from a fast food joint, and tells him to pee in that. So, my brother does as she says, and a couple seconds later starts shrieking hysterically -- "Mom! Mom! It's going to overflow!" So my mom is screaming, "So STOP PEEING!" while simultaneously trying to drive and evaluate the situation in the back seat. My brother: "I CAN'T STOP!" (Meanwhile, I was laughing so hard I couldn't breathe.)

The moral of the story is: make sure you MacGyver yourself a large enough bottle.