Slightly confusingly, the London Lite today says that the large fall in annual tube-robberies (from 399 to 192) vindicates Mayor Boris's zero-tolerance approach to knife crime. But... the fall is from 2006 to 2007, i.e. quite a few months before Boris got into office!
(BTW the fall is a good thing, but since it comes from a policy of random screenings at tube stations, it's quite likely to have simply shifted the problem out of the underground and onto the streets, surely? Let's hope not. But it means we can't really know if a more general screening programme would have the same effect...)