Many years ago, around the mid 90’s, i was driving a grab for a major utility company in Bradford. One day one of the grab drivers had a bad dose of the shkitters (runs), while mucking away in Bradford city centre.
So, when the pressure came on him he had no choice but to jump into the tipper body to have a dump. And because he was blocking one lane of the road in order to lift the muck, the traffic was a bit slow moving.
Anyway, there was he, squatting in the corner of the tipper body, straining and squeezing for all he was worth, when lo and behold, a double decker bus stopped along side him. I asked him, ‘‘So what did you do when the bus stopped?’’ To which he replied ‘’ Well, what could i do? People were looking at me, a bit bemused and probably sickened to see me. So i waved to them and smiled.‘’ And he said that some of the passengers acually waved back. 
It’s turned into a bit of an urban myth now with fellahs repeating the story throughout the industry, each saying it was a friend or friend of a friend. But the gang on site couldn’t wait to tell anyone and everyone when they got back to the yard at the end of the day.