January 8, 2018

Not dropping every year

Stuff has a story on road deaths, where Julie Ann Genter claims the Roads of National Significance are partly responsible for the increase in death rates. Unsurprisingly, Judith Collins disagrees.  The story goes on to say (it’s not clear if this is supposed to be indirect quotation from Judith Collins)

From a purely statistical viewpoint the road toll is lowering – for every 10,000 cars on the road, the number of deaths is dropping every year.

From a purely statistical viewpoint, this doesn’t seem to be true. The Ministry of Transport provides tables that show a rate of fatalities per 10,000 registered vehicles of 0.077 in 2013, 0.086 in 2014,  0.091 in 2015, and  0.090 in 2016. Here’s a graph, first raw

and now with a fitted trend (on a log scale, since the trend is straighter that way)

Now, it’s possible there’s some other way of defining the rate that doesn’t show it going up each year. And there’s a question of random variation as always. But if you scale for vehicles actually on the road, by using total distance travelled, we saw last year that there’s pretty convincing evidence of an increase in the underlying rate, over and above random variation.

The story goes on to say “But Genter is not buying into the statistics.” If she’s planning to make the roads safer, I hope that isn’t true.


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 See all posts by Thomas Lumley »


  • avatar

    Not buying into the bogus statistics is a good thing, I think!

    1 week ago Reply

  • avatar
    Megan Pledger

    I wonder if there is also some demographic factors going on as well. The crash rate went down as the Baby Boomers aged into middle age and greater caution. But now as the Baby Boomers have aged even further (~<=75 yo), into health conditions that effect their driving ability, their death rate in crashes goes up, effecting the national death rate.

    I couldn't seen any historical data of crashes/deaths by age but being over 80 really increases the risk of dieing from a crash compared to age group below (though small numbers).

    1 week ago Reply

  • avatar
    Steve Curtis

    ” Associate Minister of Transport Julie Anne Genter believes all the funding being poured into a few roads left unsafe roads through the rest of the country”

    As there has been a reduction in the number of traffic safety police, you could say the 2 are linked.
    Victoria , which is 6.5 mill people instead of 4.5 mill, has a lower total toll that us and had an decrease in total deaths recently (11%) has a much stronger traffic enforcement and more road safety measures.
    NZ seems to have made some rural roads state highways, not because they needed safety improvements but because logging trucks were wrecking them.

    1 week ago Reply

  • avatar
    Steve Curtis

    This isnt a direct quote in the story referenced above but makes you wonder about the journalist

    ‘She called an urgent meeting for pointy-headed people in the transport arena across New Zealand to come to Wellington to discuss the epidemic.’


    The wording of the press release called for ‘a road safety summit’

    1 week ago Reply

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