Wednesday, October 07, 2009

A review of the Catford South/Whitefoot North 20 mph zone

I received this questionnaire from Lewisham council asking for my views on the 20 mph zone covering the conservation area, Corbett Estate and North Downham estates (though oddly not the prefab estate, not sure why). The zone was set up in "Spring 2007".

The first thing they do is to look at the impact of the zone in terms of accidents and casualties. A laudable aim. However, some of the data are not what I would consider fit for purpose. First things first, they claim to analyse two periods of 30 months, before and after introduction of the zone. A reasonable approach for a first pass at looking at the impact of the zone. Unfortunately, if my calculations are correct, that means that the "before" period includes 3 winters (2004/5, 2005/6 and 2006/7) and the "after" period includes 2 (2007/8 and 2008/9). Since accidents are strongly seasonal, this gives a false picture, making it very likely that numbers would be higher in the first period.

Secondly, they present two tables, one of accidents and one of casualties, which split injuries into "fatal", "serious" and "slight". There was 1 fatal accident, giving rise to 1 death, in the period before the introduction of the zone and no fatal accidents/casualties in the period after the zone was introduced. Presented in the table as a 100% reduction. Well, yes, but what is more likely is that fatal accidents in the zone are so rare as to be pretty much random. A good thing, of course, but nothing to do with the zone itself. When we look at serious injuries, there were 10 before and 3 after, a 70% reduction, but again based on tiny numbers and without taking into account possible seasonality. I'm not very convinced about using these figures for a before and after analysis either. The number of slight accidents/casualties is greater, and shows a similar percentage reduction. Again, not taking into account possible seasonality.

They then go on to extrapolate savings to the community based on some method of allocating a cost of each category of casualty. They estimate a saving of £3.5 million. Unfortunately, the leaflet doesn't see fit to tell us how this was done. Clearly it has more validity if based largely on the more robust figures (i.e. those with larger numbers). I would be very sceptical if this applied any significant amount of money to the one death before (though I suspect that it does). I would be very interested in the details of the method.

What is probably more robust, and in my mind should probably be the focus of the leaflet is the measurements of traffic speeds. According to the leaflet, these have reduced in all but three roads and the average speeds range between 14 and 27 mph. However, this paragraph fails to tell us what the average speeds were beforehand, so only a cautious welcome there. There is also no explanation of the average speeds of 27 mph, though one would hope they are on the 30 mph roads that border the zone.

I have filled in my views on the relevant sheet and returned it to the council. Their analysis is clearly well intended - it makes a change for analysis to be done at all to look at the impact of policies. It's just unfortunate that it wasn't done better.

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