Association of British Drivers chairman Brian Gregory had this to say: “As with most pet road safety ideas proposed by amateur enthusiasts – speed humps, speed cameras, etc, – there is little attempt to collect scientifically sound evidence of the benefit of such ideas. No proper controlled, “double-blind” trials are undertaken…“
Brian Gregory is absolutely right. Too many of our transport policies — everything from encouraging the wearing of bicycle helmets to attempting to “smooth traffic flow” in cities — are based on weak evidence and poor quality research, and many of them would benefit from well designed trials.
In particular, a proper double-blind randomised controlled trial of 20mph speed limits would be very welcome. I’m sure you can see how it would be designed.
First we take every residential neighbourhood in the country and split them randomly into “test” and “control” neighbourhoods. It’s double-blind, meaning that the boffins doing the stats should know only that there are two groups, and be “blind” to which one the test has been applied to until after they’ve finished crunching the numbers and determined which group of neighbourhoods came out best.
Then we give all of the test subjects (that is, residents, pedestrians, cyclists, children, the local economy, and anybody/thing else expected, according to the hypothesis, to benefit) the treatment — 20mph speed limits for their neighbourhood. Except that only those in the test neighbourhoods get the active ingredient; in the control neighbourhoods, they receive an inert placebo. It’s double-blind, meaning that these subjects must not know whether they are receiving the active treatment or the placebo.
The active ingredient, of course, is reduced speeds. So we have to implement the active ingredient in the test neighbourhoods while maintaining blinding amongst the test subjects. Easy enough, of course: fit to every vehicle a simple computer which recognises when it is in a test neighbourhood and limits the vehicle’s speed to 20mph in those areas. In the control neighbourhoods, the motorists should continue driving as they usually do at 35mph.
I’m sure that’s exactly what Brian Gregory must have had in mind, and it’s great to see the Association of British Drivers campaigning for this.