20 mph speed limits and cycling

George writes "I see this as a much more practical way forward for British towns and cities than hoping for a network of cycle tracks:
http://www.eta.co.uk/blog/andrew-davis/2009/12/07/20s-plenty-comes-age?utm_source=newsletter&utm_medium=email&utm_campaign=apple"
In other words a 20 mph limit would result in increased cycling and a much safer environment for cycling and walking.
A recent report seems to support that view  - "The introduction of 20mph speed limits in areas of London has contributed towards a 50 per cent reduction in the number of children killed or seriously injured on the roads.
Research published online in the British Medical Journal examined casualty rates between 1986 and 2006 and found that in spite of a steady decline in the number of road casualties in London, the additional effect of the 20 mph zones was a reduction in casualties and collisions by an amount that has taken over 20 years to achieve on roads without them."
However similar results have not been apparent in the Netherlands. As David Hembrow reported  on his blog in May "A recent report from the Fietsberaad shows that merely having lower speed limits is not quite enough to get adequate safety for cyclists.

The Netherlands now has over 30000 km (18000 miles) of roads with a 30 km/h (18 mph) speed limit. These roads are safer than 50 km/h ( 30 mph ) roads, but not as safe as they used to be. The number of cyclists and pedestrians injured and killed on them is rising as drivers become more familiar with the lower speed limits and break them more often. In the last ten years the figures have risen from 2.4 to 11.7 serious woundings per 1000 km of road. Two thirds of accidents involving children 11 and under occur as they cross the road."

So it appears that whilst lower limits are to be encouraged, especially in towns and residential areas, they will not on their own result in more cycling and less accidents.

The Parade in Leamington is perhaps a good example of the problems associated with a 20 mph limit. When the scheme was being planned it was intended that all town centre roads would have a blanket 20 mph limit but this was quickly dropped when then police said it would be unenforceable.

Apparently if 20 mph limits are set they have to be "self enforcing". In other words lots of bumps and other features to slow traffic. If they are to be enforced by traditional means then frequent repeater signs are needed as 30 mph is the default limit. The dEpartment of transport also requires that the speed before the implementation is sufficiently higher than 30 mph - " if the observed 85th percentile speed is within 7 mph or 20% of the proposed limit, the new limit may be introduced. For 20 mph speed limits it is recommended that the 20% figure is applied. If observed 85th percentile speeds are above 24 mph, then it is unlikely a 20 mph speed limit would be appropriate, unless traffic calming measures can be provided."

SO with minimum humps or other slowing features the speed is often above 20 mph and the police will not enforce.

Other problems with such a limit seem to me to be that fit sports cyclist will often exceed 20 mph so can be slowed by cars and an everyday cyclist may ride at 10 mph so the speed differential is still great and being hit by a tonne of car at 20 mph will still hurt - a lot! 

So by all means press for 210 mph but what we really need is decent cycling infrastructure.