bus

Most of the time you just spell it, (VERY quickly).
To sign "bus driver" combine the concepts BUS+DRIVE+AGENT.

bus:
Not everybody knows this version:

Sign: bus
A third version is to mime the driving of a bus. Hold on to a very big steering wheel, puff your cheeks out slightly, and turn the wheel back and forth a couple times. This version tends to show up in stories after you have identified the bus and are indicating how the bus is being driven.