do you mean slower or faster?
if you mean slower, because out of THAT LONDON (yes there are places you know) most 30s are small villages and are a complete waste of time. 20 years ago I was caught doing 49 in a 30 on the edge of a little village in Staffordshire. Nowadays you might get banned for that, regardless of context. Then I was let off because, and I quote Her Majesty's Constabulary "It's the most stupid 30 zone in the county, but we have to police it because the rich locals complain unless it's them speeding. Have a nice day"

Context is the problem. We broadly apply speed limits and then enforce them for revenue, not safety reasons. It's stupid, pathetic and counter-productive in safety stakes.
( the mighty badger Aphrodite, on a bar stool, by your side, Wed 13 May 2009, 10:01, archived)

Never mind THAT LONDON
I gave up driving when I got here. I drove around small East Yorkshire villages, and it was a bugger to drive safely past 20.
(rnuk http://flickr.com/rnuk, Wed 13 May 2009, 10:03, archived)

that's because half of them have never seen a car before

( the mighty badger Aphrodite, on a bar stool, by your side, Wed 13 May 2009, 10:05, archived)

There is this.
When buses drive around Barnsley, the locals come out and punch it because they think it's a big metal cow.
(rnuk http://flickr.com/rnuk, Wed 13 May 2009, 10:11, archived)