Sadly, we can't even blame Boris (but it was in his manifesto). TfL did a pilot bit of research (Pistanator is right, it was the A23) in 2006 on it, and then some more in 2007, probably cos the 2006 research didn't come up with the right answer. There is some figure being bandied about that it reduced motorbike related accidents.

What's more worrying is that those 'licensed mini cab'* people might get access to it soon too.

* ie blind cnuts