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I hadn't realised the police refused to enforce them. I live opposite a school on a busy rat run, 20mph obviously, and cars turn into the junction at the end of the road, metres from the school, so fast sometimes they can't keep left and go round the other side of the traffic island - i.e, they enter on the wrong side of the road. Next to a school. A teenager was hit there a couple of months ago by a woman going too fast (though on the right side of the road I think), badly injured, and the cops stationed a car further up the road for the rest of the day, presumably to look for anyone speeding. I haven't seen them since. What a shower of shit.
Sharkstar
@Oliver Schick
As I think I've said before in various places, 20mph limits aren't very significant achievements. I mean, I support them, but they became an 'easy to achieve' policy somewhere along the line. In London, the significant step came simply because councils went ahead with them despite the police saying they wouldn't enforce them. Prior to that, the lack of police enforcement had always been the argument that killed any campaign for them, and councils continued to go ahead with 'self-enforcing' schemes such as vertical or horizontal deflection. These worked quite well for a time, and certainly contributed to casualty reduction, but essentially had had their day when car suspension started to become much better and there started to be so many 4x4s.
I still see 20mph as a good kind of policy, but it needs the other two key policies to go along with it to work--modal filtering and on-street car parking control/reduction. Filtering should be done in such a way that the end result is a coarse-grained network (coarse-grained permeability) for motor traffic and fine-grained permeability for walking and cycling. This means filtering not at the edge of traffic cells (bounded by main streets) but as close to the centre as possible, which usually requires some kind of arrangement for public service vehicles like dustcarts, e.g. keys to gates. The deterrent effect to drivers is achieved by lots of effective cul-de-sacs for drivers, few continuous alignments, and same way in-same way out arrangements.
In a lot of places, engineers will attempt to filter at the edges of cells, which is a mistake. All it usually does is to remove one or two rat-running alignments while diverting rat-runners onto others. Well, those, too, may be filtered in the same way in due course, but the end result is a cell which is effectively still permeable for drivers, e.g. in loops, and that doesn't help much with local driver behaviour, e.g. speeding. Also, filtering at the edges increases motor vehicle capacity on main streets, as it removes more junctions from the main streets. It's important to have as many side street junctions maintained as normal, functional junctions, to act as natural checks on driver behaviour there, even if volumes from such side streets are low because of the internal filtering.
Anyway, tl;dr--20mph isn't effective without companion policies and has become an 'easy to win' policy because councils don't care any more that enforcement isn't built into its implementation.