I understand there are 27 in each direction (a touch of 'overkill'?). The Airlinks busses are now running about 5 minutes late on all journeys and their drivers reckon they cross about 1500 of them in a day! I suspect there will be a few 'industrial injury claims' forthcoming. Apparently their busses miss the humps with the front wheels but the double set at the back catches them, not to mention the cars in the middle of the road.
Speed there does most definitely need to be controlled but both attempts by BAA seem to be misguided? Why not speedcams? Should make a fortune.