More on why 20's plenty -- without need for any further 'trials'...
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 value of conducting any trial we must ask ourselves some serious questions first of all.
1. Is there a need for a trial?
If there is already ample evidence that the policy being trialled is effective, then there is no benefit from conducting a trial. In the case of 20's Plenty there is ample evidence that this works. This is both the norm for European towns and has been implemented in many UK towns in far wider local areas than those currently being planned in the "trial". One wonders whether Norwich City Council will learn anything new from a poorly designed trial that it could not learn from published material and evidence that already exists.
2. What are we trying to understand from the trial?
If one is conducting a trial then it is necessary in advance to plan exactly what one is measuring and what actions will be taken as a result. This planned trial does not seem to have any such preparation. The idea that a monitoring strategy will establish the potential casualty reduction benefits for all road users is rather optimistic. In order for this to be beneficial one would need to plan a city wide implementation of a default 20 mph limit and then establish trial areas where one is accurately modelling the larger initiative. This should take account of the type of roads, handling of gateways and traffic calming, police enforcement and public education that had been envisaged for the city wide implementation. Hence without developing a plan for implementing 20's Plenty throughout the city and understanding how to scale this down so that the trial is realistic then no benefit will result from the trial.