You have to react. Anyone with a functioning set of reflexes will react by being startled.
Personally, when this happens, I sit up on the bike and do the full 'self-gratification artist' sign - one hand behind the head, other mimicing the shuffle - because they will see it in the RVM and they will be in awe at my bike control.
Personally, when this happens, I sit up on the bike and do the full 'self-gratification artist' sign - one hand behind the head, other mimicing the shuffle - because they will see it in the RVM and they will be in awe at my bike control.