Tuesday, October 5, 2010

Shock Horror! 20MPH zones reduce crashes!

The Pope is apparently Catholic and bears sleep in the woods, according to Ross Lydall.

Or in the statistically illiterate Mail or Telegraph, black is white and sewage flows uphill.

These are two different spins on a report on the Portsmouth 20MPH zone prepared by Atkins for the DfT.

I'm going to ignore the Mail and concentrate on the Telegraph, as it considers itself a serious paper, although the tone of both articles are similar.

Unfortunately, the Atkins report itself is contradictory. In the Executive Summary (P1), it states "early figures suggest that the implementation of the 20 mph Speed Limit scheme has been associated with reductions in road casualty numbers", but on P36 it states "Based on the available data for two years after scheme implementation, casualty benefits greater than the national trend have not been demonstrated". It is rather sloppy of the authors not to explain the dissonance between these two statements. However, the casualty statistics tell their own story: there was a 22% reduction compared to a 14% nationwide trend over the same period in comparable areas.

The Telegraph has got a medical statistician, Professor Stephen Senn, to criticise the report, "The design of the report is very bad. Various statistical terms are used incorrectly and they've probably used the wrong statistical test. They haven't got a control group, which is pretty basic, and [sic] without which it is pretty naive to jump to conclusions." Note that he did NOT say that "20mph limit has not made roads safer", which is the Telegraph headline. As the Prof says, " it is pretty naive to jump to conclusions". Right back atcha - in your face, Telegraph!

The Telegraph also wheels out Idris Francis, who is best known for driving at 47MPH in a 30MPH zone, and attempting to use the 'right to silence' to wriggle out of the conviction. Not exactly a road safety expert then, no relevant statistical qualifications as far as we know. Just someone who likes driving fast.

However weak the Telegraph's case is, I am rather sceptical of the value this kind of study. The laws of physics tell us that speed increases the probability and severity of crashes, and there is a long history of empirical confirmation of this fact. On the other hand, a street is a complex, dynamic system where there are many interacting behaviours which increase or reduce the probability of crashes.

For example, the study says "there was an 11% increase in the number of KSI pedal cyclists". It could be that more people are cycling as a result of the 20MPH limit. Or it could be that cyclists are expecting motor vehicles to be travelling slower and taking risks base on that expectation. We simply don't know.

What is really instructive is to look at the accident location maps in Figure 5.1 on P16. Consider that each of the dots on the map is someone's life - a real person, not a statistic. Look at the density of dots on the 'before' map, and on the 'after' map. Now tell me that the roads aren't safer with the 20MPH limit.

1 comment:

  1. Portsmouth kept all A-roads at 30, including the seafront (one cyclist killed there) and the main shopping streets. what they have done well is drop the Motorway speeds gradually from the 27, to 60 mph as the M275 begins, down to 40 closer to town.

    A city wide 20 mph zone, rather than just all side streets, may have different results as the main roads should not only have lower traffic speeds, it will be easier to cross the side roads by them


    Incidentally, why is that the Mail and Telegraph are quite happy to cite anything statistically weak that proves the speed cameras are dangerous, yet discount all statistical correlations related to things like global warming? It's as if they only choose to believe the conclusions of those studies they agree with, and the others they just argue about the maths.