There are a number of reasons for setting a default speed of 20mph throughout the 'unclassified' road network in residential parts of a city such as Norwich. Most important perhaps is that of driver recognition as to what the speed limit is. If you have 20 mph as the default then you not only establish this limit clearly, but you also adjust the driver's recognition as to what is the "normal" speed in such residential areas. The most important aspect is that drivers set 20 mph as the norm with those 30 mph areas or roads being seen as 50% faster, rather than seeing 30 mph as the norm with 20 mph as 33% slower. The psychological adjustment of normal speeds in residential areas is key to the success of reducing speeds and therefore danger to pedestrians and cyclists. Therefore any trial, pilot or whatever, that only implements 20 mph in a tiny proportion of the city is not modelling 20's Plenty in any realistic manner. When considering the val...
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[For some further reflections in a more critical vein, see also http://rupertjason.wordpress.com/2009/07/28/norwich-north-some-reflections/ ]