I can also recall this incident very clearly. It was when I was 13 and in secondary school. We were going on a day trip to what used to be Windsor Safari Park (It’s now called Legoland, I believe).
It was a wet and windy day as I recall, and I must have been desperate to pee several times that day. When I got to school, I felt I needed to go to the toilet. Before I had to board the coach to go on the trip, I visited the toilet. I was relieved to have gone.
The coach was already traveling along the motorway when I felt the need to go again. It must have been the windy weather that made it cold. Unfortunately, the coach didn’t have any heating inside.
When we reached the safari park, I was bursting- I needed to go, but as we reached the gates, I knew we were not able to leave the coach for at least an hour or so. As the coach moved slowly past the animals, my bladder was filling up again, which was a puzzle to me, because I hadn’t drank anything beforehand.
I was paying too much attention to holding my pee rather than watch the animals. At the end, I felt I was on the verge of wetting myself. I was holding my penis and crossing my legs using all the willpower that I had.
I eventually made it in the end. When the coach reached the outer buildings, I was the first to get off and raced to the toilet. I relieved myself and made sure every last drop of pee came out of my penis. Funnily enough, I was not desperate again until I got home from the trip!