It's a tricky one, this. Clapham High Street, and indeed Clapham's regeneration can be traced back to the moment the Clapham Picture House opened its doors. There wasn't such a seismic event in ED recent history. Word of mouth from new residents must've played a big part. That's how we heard it was a great place to come.
I think Les Alden's first point goes some way to answering this. A small independant, with no brand and less cash at their disposal, is more likely to follow the rules.
Barnet council got rid all theirs, much to the annoyance of Ken Livingstone, who tried to withold council funding as a punishment. They saw a 14% drop in casualty rates.
"Edit: ooh actually this says that "ambulances and fire engines will be able to cross the device with little discomfort, irrespective of their speed". Which means so will transit vans and bin lorries. Better than normal speed bumps though.