A double-blind trial of 20mph speed limits is an excellent idea

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.

9 thoughts on “A double-blind trial of 20mph speed limits is an excellent idea”

  1. Maybe he had rigorous enforcement of 20mph speed limits in a parallel universe in mind? After all member of the Association of Bad Drivers do seem to think the are living in a parallel universe, one where the Laws of Physics can be bent when ever they are found to be inconvenient…

  2. Perhaps he means we need to compare UK road danger to road danger in somewhere like, say, The Netherlands?

    The UK population are almost entirely blind to the fact that civilised nations have lower motor speeds in streets, and the NL population are similarly blind to the concept that streets can be so dangerous and that people would be allowed to drive at 30% higher speed in a civilised country.

    If so, it should be a very cheap experiment to run, as the data is mostly already collected and published. We’d just need to anonymise the data sets to Mr Gregory couldn’t tell which country they were for, and he could decide which country had a better road safety record: country “A” or country “B”.

  3. I’m quite impressed at the progress the ABD has made here, they seem to be suggesting that motorists’ cars be fitted with some sort of system which forces them to comply with the speed limit in certain areas, for the purpose of this study at least (even if the readout on the speedo would have to show what speed the driver was trying to do)

  4. Just heard a very interesting article on Radio 4’s ‘More or Less’, about twelve minutes in. Check it out.

    In 2009, Bristol had fewer than 50 roads with a speed limit of 20mph. But during 2010, the council launched two pilots, one in May and one in October, lowering the speed limit to 20mph on around 500 roads. These were ‘signs only’ – so no speed bumps – but even so, average speeds have dropped, walking and cycling has increased, and 82% of the people who live in the pilots think they’re a really good idea.

  5. Same was true in 2009 and 2010 following the introduction of a city-wide 20-signed limit in Portsmouth. Apparently there has been an increase in accidents there recently but the numbers involved are too low to be statistically significant and they can’t be meaningfully compared with a control sample to see whether other factors are in play.

    As for the ABD, it is interesting to see how they now demand a proper scientific trial, even if they have confused terminology from pharmaceuticals testing. They seemed happy enough last week to leap to conclusions from the raw data on total casualties in 20 limit streets without considering whether they should be factored by, for example, an increase in the number or aggregate length of such streets. A bit like someone pointing out (I odn’t recall where I read it – here?) that the number of motorway accidents has soared since the mid 1950s.

    I thought “More or less” did a workmanlike hatchet job on the claim, sadly made also by another arm of Auntie herself. Pity it didn’t merit an airing on Horizon instead.

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