I've used bus racks a good bit and never had a problem. Even on some really awful bumpy roads, they are remarkably secure.

There was also one moment that I'm not terribly proud of; it had been a long day at work, and I hadn't gotten much sleep the night before, so I took the bus back from work. Only, I fell sleep on the bus, woke up one stop too late, rushed off the bus, and forgot that I had a bike. My bike rode the bus all the way around, and back to the terminal. After a good nights sleep, and a very confusing morning, I realised what I must have done, I called up the bus company and apparently it was just sitting at the depot. I picked it up the next day, no trouble at all.