The London Metropolitan Police has a programme where they stop and search people for weapons.
These searches were even more successful "when people give us information into who we should be stopping", according to Met Commissioner Bernard Hogan-Howe. He added that when someone regularly carried a weapon, their "brothers will know, the girlfriends will know, the people they go out with every night will know...If they tell us, we'll make sure we stop those people, and therefore make our stop-and-search even more effective."
I bet that gives the US 2A crowd the willies.
Personally, I preferred the knife exchange programme.