Thursday, September 13, 2012

Road safety - how not to reason from data

My apologies for going quiet over the summer.  I've been in China, and they're not too big on blogs there (blogger is blocked).

Just a short post here, on something fairly obvious.  Here's an extract from post number 51 by Cambridge News' Cycling Blog on speed limits.
In Great Britian in 2011, 7 people were killed on a road with a 20 mph limit. 636 killed in a 30 mph limit. 289 people were seriously injured in a 20 mph limit, 13,168 in a 30 mph limit... Yes, these are large numbers. But it is the proportions that matter here... So that is proof then. Lower speed limits means fewer people killed.
[In case you're wondering, there's no irony in the last sentence.  At all.]  I assume you're all thinking the same as me by now.
  1. There are a lot more 30mph roads (by length) than 20mph ones.
  2. 30mph roads are often busier than 20mph roads, so there's more things to collide with each other.
After a cursory glance for official proof of the first point I came up empty-handed, but I imagine you'll believe me.

There are a lot of good reasons to think that 20mph roads are safer than 30mph ones, at least if the speed limits are observed, just from simple physics.  A car travelling at 30mph has 2.25 times as much kinetic energy as one travelling at 20mph.  As a driver reacts before braking, the car travels 50% further; as it starts to brake, this will happen more slowly the faster the car is going to start with.  I am personally in favour of 20mph limits - they're a lot more pleasant to cycle on, because I can actually keep up with the traffic.  They certainly feel safer, because drivers are less likely to try and overtake me inappropriately.  But what's listed above isn't proof of anything.


  1. A related issue of 20mph speed limits was reported on More or Less a couple of months ago.

    The episode is here:

    1. Thanks for that Matt! (I have to start checking the comments section more often...)