Please report dangerous drivers to the police - Roadsafe. Report to plod, not just whinge here.

I would love to know what happens if you attempt to citizen arrest a bus driver. I can't see that going badly at all.

Obviously depending on the level of offence/threat will ultimately decide what action I take, if the police can get there then it wont be needed.
Common assault is defined thus:

Common assault is committed when one person causes another to apprehend or fear that force is about to be used to cause some degree of personal contact and possible injury.
Just because that force is from a vehicle (even more potential damage/injury in fact) makes no difference.
A citizen's arrest or power of arrest can be justified if the assailent is clearly threatening or has actioned the assault.