Tuesday, 8 June 2010

Good “road toll,” bad statistical analysis [updated]

Now that we’ve got some good “road toll” figures for a holiday weekend to shout about, can we try to finally get our heads around the fact that the figures from one weekend—whether good or bad—tell us nothing at all about either the efficacy of road policing policies, changes in driving habits, or improvements in the state of the roads.

Whatever road policing bigwigs might now try to tell you about the success of their ‘zero-tolerance-for-speed’ policy over the weekend, fatality figures from just one weekend are statistically insignificant.  Whether good or bad, the sample size itself is so low they tell you nothing at all—not unless you’re a bad journalist keen for a colourful headline, or a traffic policing bureaucrat eager to cement in a new policy. Like Paula Rose, for instance, who “is amazed by what the extra police resources* on the roads and the 'no excuses' speeding policy have done.”

Did their new policy cause the statistics? No, they didn’t. Correlation is not causality, however much the cheerleaders for slow driving might wish it otherwise.

* * * *

* Extra police resources?  I’m not sure there were any more “police resources” out this weekend than on any other holiday weekend; and since in driving north from Auckland on Friday night I followed one car with only one headlight and passed another, I doubt that those “resources” were any more vigilant than they normally are. What was your experience?

UPDATE: Zen Tiger does a bit more analysis.


  1. I suspect the wet and cold weather over most of the country, meaning more people stayed indoors rather than venturing out, was a more likely contributor to the low road toll.

  2. My experience was that there were cops all over the place, most having stopped some poor sod for some presumably insignificant infringement, and that there were the usual inconsiderate morons who dribble along at 80kms on the main roads, but speed up to 110 when the passing lanes appear.

  3. PC

    The police publicised that the tolerance range for drivers who speed would be lowered to around 4 km per hour over the speed limit for the holiday weekend.

    Then once a lower road toll was achieved, they reward all drivers by telling us that they will seek to make this lower tolerance permanent. Thanks so much!

    Seems to me that whichever way the road toll went, they would have sought to reduce the tolerance range permanently.

    Note to the police: Going over the speed limit may not - in fact - be dangerous driving. A good driver knows what speed is appropriate for the conditions.


  4. My colleague was snapped from behind by a speed camera, 2/3 of the way along a passing lane, on a straight stretch of road, doing around 120km/h.

    On the motorway you can overtake slow traffic without exceeding 105 km/h. On the open road, such behaviour at passing lanes will result in only 1 or 2 vehicles getting passed. Consequently, frustrated drivers will risk overtaking at more dangerous locations.

    This policy will backfire in fatal crashes and further erosion of respect for police.

    (BTW, I agree 100% with your criticism of official use of road safety statistics.)

  5. I heard, radio interview on Radio Live, the number of crashes was also down but not to the extent fatalities were. Therefore the statistics the Police take credit for were nothing but luck.
    What makes a crash fatal? Not +5 km/h

  6. I know...when they announced it, I told people I was almost hoping for an increase in fatalities, because that's the only way they're not going to consider this policy the cause and make it permanent...

    (My solution for reducing the road toll: increase the speed limit, enormously: when was the last time you heard of a road accident at, say, 300km/h? Never, right? Proves it's safer to drive fast!)


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