The Doctor wriggled in his seat on the bus. Then he crossed his legs. Donna noticed, and asked, "Do you need the loo?"
"A bit", he answered.
"Well, we'll be at my mum's place soon, then you can go" she assured him. He nodded and shifted his focus to the window to gain some patience.
A few minutes later, the bus stopped, but not at a bus stop.
"Why have we stopped?" the Doctor asked, looking ahead of the bus. There was a gridlock of cars in front of them. The bus driver announced what was obvious; they were stuck in traffic.
Both Donna and the Doctor committed to staying optimistic, despite the Doctor now wriggling with crossed legs.
The bus eventually started moving again. A couple stops later, Donna looked over at the Doctor. He was openly grasping his crotch, both legs slammed shut, and shaking with need.
"Just hang in there, alright? Just about 5 more stops left." Donna tried to reassure him.
"I don't think I can, Donna. This is really pushing the limit. Don't think I can wait much longer," the Doctor replied, now completely panicked.
Knowing what would happen if they stayed on the bus, Donna lead the Doctor of the bus at a stop she knew had a street with shops.
They both looked around for a shop that was likely to have a public bathroom. The Doctor spotted a Starbucks, and ran in. As is true most places, the Doctor was fortunately told the restroom was free to use. Donna saw the Doctor running happily to the bathroom, through the window.
It was a single-person bathroom. The Doctor smiled as he stood in front of the toilet, unzipped, and started urinating, to the Doctor's great relief.
I really need to start remembering to wee before going on public transport, the Doctor thought to himself.
He finished and returned to Donna, smiling brightly and feeling much better. They had a nice 20-minute walk to Sylvia's house. The End.