gallyslave; This also annoys me.
There is a sign in a local bus lane ,which states very very clearly "Bus lane NOT in use".
Yet when I go into it,the other drivers think I'm trying to get by them and freak!
They might be freaking at you because you are "undertaking" them which is illegal and maybe they are also unaware that the bus lane is actually open for anybody to drive in, but the road marking would suggest another thing