Long ago I spent a few months driving a car rental bus. I'd say about 20-30% of the customers tipped. Some people did it if you helped with the luggage, but a lot of the tipping seemed to be based on how long the customers had to wait. If I waited for 10 minutes for a full bus load, it was not uncommon to get no tips. On the other hand, if someone got on the bus, and I took off right away, I often got a tip. Some of my best tips came when people were running late and I left right away, and moved right along. In general, it also seemed that the less full the bus was, the more likely people were to tip.