'Self-enforcing' 20mph zones, with chicanes, tables, humps etc., are effective at reducing overall casualty rates. It's been widely statistically proven. However, as a cyclist I'm certainly aware of zones whose implementation is seriously flawed. Apparently a well designed zone should be self-policing with minimum enforcement. There are several near me where the infrastructure does little to control speeds, the vast majority of drivers continue to travel in excess of the previous 30mph limit, the calming measures offer little impedance to vehicles travelling in a straight line at 40mph+. In several areas the traffic calming elements can be easily by-passed, not sensibly or legally maybe, but that isn't an obstacle to many intent on 'making progress'. The trouble I have as a cyclist is that these 'calming' measures while failing to control car speeds present a hazard to two wheeled vehicles. These are easy enough to deal with, with good anticipation and road positioning, isolated from the actions of any other traffic but introduce a driver committed to maintaining their speed through the 'calming' and it's another matter. I now have several points on my regular route where I can expect motor vehicles to mount the pavement, pass to the right of bollards and mini round-a-bouts or run through cycle by-passes - all at speed. The standard issues of racing to pinch-points and close passes seem to be exacerbated by drivers' need to get through the zone as quickly as possible. On-coming traffic can be expected to swerve to the wrong side of the road at every bump.
Apparently enforcement of 20mph zones in Southwark is one of the priorities of the local safer neighbourhoods team. Well, they and the council say it is, in fact they do bugger all.
Taking newer cyclists out on local roads often reveals that, contrary to their expectations, the 'calmed' 20mph zones are far less pleasant places to be on a bike than the local trunk routes. The leafy, suburban village of Dulwich is far more cut and thrust than the churning thoroughfare of Brixton Rd. There may be more, louder, larger, faster traffic in Brixton but it feels less of a threat and behaves far more predictably.
It's becomming more common around here. Stronger cyclists get to a 20 limit & put the hammer down. Why? Because sitting at around 13-17mph is an absolute nightmare in a 20 limit when motorists are doing 23-25mph. They hand beside you for a long time & then squeze you as the inevtible car coming the other way aproches. You either get up to car speed & sit in the off-side wheel track or slow down to about 5-8mph.
That chimes exactly with my experiences in South London. It's a joy on those rare occasions I find myself either behind or in front of a sensibly driven car and can roll through without molestation.
20mph zones are enforceable in law, but as it is a matter of policy from Westminster down that the zones should be designed such that enforcement isn't necessary we seem to be shafted when their implementation falls short.