There’s a growing movement in the UK – and around the world – to reduce urban speed limits. New York City recently reduced their limit to 25 mph, to quite a lot of chatter. Some UK cities are considering extending small 20 mph pilots to cover larger areas.  Is it something we should be supporting?

Of course, a few moments of simple arithmetic will give you ample reason to support this. If I take the bus at the wrong time of day – the profoundly miss-named “rush” hour – it can take me an hour to travel the four miles from home to work or back. That’s an average speed of 4 mph. In a simple minded model, if we assume that I spend as much time as possible at 30 mph, then that’s 8 minutes; the remaining 52 minutes I’m stationary. Reduction of that top speed is not going to have an appreciable impact on my journey time. Assuming I can travel at 20 mph, then I prolong my journey by 4 minutes, all other things being equal. Of course, this is a gross oversimplification, but it does serve to emphasise that in urban areas, peak speeds don’t really influence your arrival time.

However, in terms of the quality of the urban environment, the top speeds of vehicles do make a difference in a number of other ways. There’s the pollution from accelerating and deceleration. There’s the impact of being struck by a vehicle doing 30 rather than 20 mph. There’s the perception of risk, with families – and in particular children – being able to walk, cycle or scoot. There are benefits also for traffic itself – traffic merges better at lower speeds.  And there are many, many other possible benefits. So, we ought to be able to prove that it’s a good thing, right?

A recent paper, described as an umbrella review – which I’d not heard of before, but apparently is a standard way of looking at these sorts of things – concludes that it’s a good thing. You can find it here: http://jpubhealth.oxfordjournals.org/content/early/2014/09/28/pubmed.fdu067 (paywall for anything more than the abstract, I’m afraid.)

I’ll admit that I struggled with this at first. My bias – as you can probably tell – is that reduced urban speed limits are A Good Thing. However, an umbrella review is a systematic review of systematic reviews – a finding which didn’t really fill me with confidence. It felt rather recursive, as did the fact that this built on a previous umbrella review. I’m not really able to judge the quality of this study, but it does give me some helpful confirmation bias; to quote from the results section of the abstract: “…measures are effective in reducing accidents and injuries, traffic speed and volume, as well as improving perceptions of safety in two of the studies. There was also evidence that such interventions are potentially cost-effective. ” They were unable to demonstrate their hypothesis that it might help address health inequalities, but they conclude – perhaps inevitably – that more research is needed because this hasn’t yet been looked at properly.

What can we do with this information? Well, it seems to me that we’ve got a role as paediatricians, which is to get behind these sorts of interventions. And also, we’ve got a role as individuals. I remember once being told to think of my driving style “as if I were a moving speed bump”. What this was intended to convey was that I could, in my own behaviours, influence road speed for more people than just me.

– Ian Wacogne