How to design a city?

Yes, drivers will exceed speed limits, but they won't ignore them entirely, as they will drive at what they consider to be a defensible (in their view) degree to which they exceed the limit. 20mph works.

But if the aim of the 20 limit is to slow them down to 30, why risk criminalising people? Why not just enforce the 30 limit?

The problem is that again, these things are done because they demonstrably reduce casualties.

I read that the London Ambulance Service wanted them all removed because by reducing response times, forcing ambulances to avoid certain roads, etc, they cost more lives than they save. If that is true there is no justification for them at all.

I suspect that with advances in car suspension probably partly triggered by just these vertical deflection measures, the percentages must now be lower.

I'm not aware that there have been any real advances in car suspension since the 1960s. normal cars get wrecked by speed humps. Which, as a result, means that speed bumps are directly responsible for the massive increase in big off-roaders in cities, because of their ground clearance. I don't know if that genie could be put back in the bottle by removing humps, but putting them there definitely killed the small city car in favour of the SUV.

Well, as ever, (chorus) 'if people weren't breaking the speed limit in the first instance, they wouldn't have to slam on the brakes' (/chorus). :)

You still get this when obeying the speed limit. Have you ever tried going down a road with speed humps at a constant 20mph? It is impossible. You have to brake then accelerate.

As far as I'm aware, the 'accident' argument is an urban or rural myth one dreamed up by the anti-speed camera lobby.

I have seen it happen in front of me on the A4, where a new camera was installed just before the flyover on the eastbound carriageway. A Mercedes panic braked, skidded across 3 lanes and crashed into some cars. One view is that he should not have been exceeding the speed limit. My view is that on this stretch of road the speed limit is far too low (40mph), and, in any case, a 6 lane dual carriageway is no place to be enforcing an arbritary speed limit.