Safer Speeds – The Real Data

I have commented before on how Transport for London (TfL) have failed to justify their “Safer Speeds” proposals which includes imposing 20 mph speed limits on many roads. We have previously pointed out how TfL have been misinterpreting police accident data to support their claims that the measures are justified.

For example, they issued a Tweet that said “speed accounts for 37% of all death and serious injuries” in road accidents in London. That figure is simply wrong. The claim was allegedly based on the STATS19 data reported by the police (a form they fill out about every accident involving injuries).  That form allows for multiple factors to be recorded and after submitting a Freedom of Information Act request we learned that they counted all the accidents where factors 306, 307 and 602 were noted.

But factor 602 is described as “Driver/rider either behaved in a negligent or thoughtless manner or was in a hurry….”. Clearly the key word in that sentence is the second “or” when TfL have interpreted it to mean “and”.  There is no basis for claiming that all accidents where factor 602 is attached were rated by the police as ones where a driver was in a hurry. They might have simply been careless. Only where the other factors 306 or 307 were also noted could there be any claim that speed was a factor in the accident.

We now have the complete accident data and the data makes it plain that exceeding the speed limit (factor 306) is a very minor factor in KSIs (Killed and Serious Injuries) in London. It’s actually recorded as a contributory factor in only 5.9% of such accidents in the last five years. That’s actually less than the figure of 7.1% reported by the Department for Transport for the national figures in 2018 – see table below. Clearly tougher enforcement of speed limits is therefore unlikely to have much impact on the overall numbers. That of course is particularly so in London where average traffic speed is typically well below the speed limit.

Contributory Factors and Speed - ras50008

The largest contributory factor by far is “Failed to Look Properly” which accounted for 42% of KSIs in London or 35% nationally. But there are several other factors with higher ratings than “Exceeding the Speed Limit” such as “Poor Turn or Manoeuvre”, “Failed to Judge Other Persons Path…”, “Loss of Control” and “Careless, Reckless or in a Hurry”.

Even if you bundle factors 306 and 307 together only 12% of KSI incidents are included nationally so reducing speed limits is going to have only a small contribution at best to reducing such accidents. It’s reducing the other factors that is the key to substantially reducing road casualties. More driver education, improved roads and research into saccadic masking may be productive.

Note also that a lot of the reported factor 306 and 307 claims of excessive speed and speed above the speed limit might well involve illegal use of vehicles such as stolen vehicles so reducing speed limits will have negligible impact in reality.

There is simply no cost/benefit justification for the Safer Speeds proposals as pointed out in our previous article and TfL have clearly been abusing the data so as to make spurious claims.

Roger Lawson


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1 thought on “Safer Speeds – The Real Data

  1. As nobody has been killed or injured by a stationary vehicle, then all accidents are ‘speed related’ !
    Get rid of all speed limits, then there would not be any ‘speeding’ accidents !
    By downgrading 30mph to 20mph, your get more ‘speeding’ accidents, raise 30mph to 40mph and you would have less ‘speeding’ accidents.
    It is just a numbers game and bears hardly any relation to the actual cause of the accident, which could be too fast for prevailing conditions, failure to judge the situation, hazard perception, not looked properly, etc etc.
    Many causes or combination of causes, just speed is easy to measure.

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