Answer: when oncoming traffic brings everything to a standstill.
Today was the final training session for my young apprentice conductor and we were confronted with some useful real-life dramas. She was in charge of the bus (under my supervision of course) and handled the platform and the passengers well. With an almost full load we had just started attacking Edgcott Hill in first gear when we were confronted by a string of cars coming down the hill. The driver of the lead car tried her best to make room to pass in the narrow lane but came to rest with the front of her car buried in the hedge and the rear sticking out at an angle, effectively blocking our way. Looking over the rail on the top deck, we (and about 30 top deck passengers) watched as the driver tried to shunt herself clear. I could see it was never going to work so I ran down the stairs and, after checking that the driver had the handbrake on, dinged the bell once and jumped off the platform. I directed the flustered lady at the wheel to turn hard right and bring the front of her car down from the hedge. The back of her car then swung in just enough for us to pass so I hopped back on board, gave the driver two bells and we were away. Still in first gear but making progress nonetheless.