May 9, 2014

Terrible, horrible, no good, very bad month

From Stuff

The road toll has moved into triple figures for 2014 following the deadliest April in four years.

Police are alarmed by the rising number of deaths, that are a setback after the progress in 2013 when 254 people died in crashes in the whole year – the lowest annual total since 1950.

So far this year 102 people have died on the roads, 15 more than at the same point in 2013, Assistant Commissioner Road Policing Dave Cliff said today.

The problem with this sort of story is how it omits the role of random variation — bad luck.  The Police are well aware that driving mistakes usually do not lead to crashes, and that the ones which do are substantially a matter of luck, because that’s key to their distracted driver campaign. As I wrote recently, their figures on the risks from distracted driving are taken from a large US study which grouped together a small number of actual crashes with a lot of incidents of risky driving that had no real consequence.

The importance of bad luck in turning bad driving into disaster means that the road toll will vary a lot. The margin of error around a count of 102 is about +/- 20, so it’s not clear we’re seeing more than misfortune in the change.  This is especially true because last year was the best on record, ever. We almost certainly had good luck last year, so the fact that it’s wearing off a bit doesn’t mean there has been a real change in driver behaviour.

It was a terrible, horrible, no good, very bad month on the roads, but some months are like that. Even in New Zealand.


Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient

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  • avatar

    I’m glad you wrote this post. It means I don’t have to do something similar to vent.
    see also: fixation on roadtolls over even smaller sampling periods, such as Easter.

    4 years ago

  • avatar
    Megan Pledger

    From memory, the weather last year in March/April was very good. The weather this March/April has been appalling – in Wellington, it’s been raining nearly every day. So, wet surfaces and low light conditions are probably not helping.

    4 years ago

    • avatar
      Thomas Lumley

      Good point. It wasn’t true for Auckland, though — March was unusually dry and April was average for rain.

      4 years ago