If you look real careful, you'll see an outline of a bus on those blue signs.
Could be wrong (it's happened before...in 1997) but that may indicate that they're bus signs. And I think they put them up high because that means people can see them from a distance. Which is clever, 'cos if they were low, only cars'd see them, which would serve no purpose at all.
After all, what car you ever met needed a bus?
:P