January 2, 2014

Toll, poll, and tolerance.

The Herald has a story that  has something for everyone.  On the front page of the website it’s labelled “Support for lower speed limit“, but when you click through it’s actually about the tighter tolerance (4km/h, rather than 10km/h) for infringement notices being used on the existing speed limits.

The story is about a real poll, which found about 2/3 support for the summer trial of tighter speed limits. Unfortunately, the poll seems to have had really badly designed questions. Either that, or the reporting is jumping to unsupportable conclusions:

The poll showed that two-thirds of respondents felt that the policy was fair because it was about safety. Just 29 per cent said that it was unfair and was about raising revenue.

That is, apparently the alternatives given for respondents combined both whether they approved of the policy and what they thought the reason was.  That’s a bad idea for two reasons. Firstly, it confuses the respondents, when it’s hard enough getting good information to begin with. Secondly, it pushes them towards an answer.   The story is decorated with a bogus clicky poll, which has a better set of questions, but, of course, largely meaningless results.

The story also quotes the Police Minister attributing a 25% lower death toll during  the Queen’s Birthday weekends to the tighter tolerance

“That means there is an average of 30 people alive today who can celebrate Christmas who might not otherwise have been,” Mrs Tolley said.

We’ve looked at this claim before. It doesn’t hold up. Firstly, there has been a consistently lower road toll, not just at holiday weekends.  And secondly, the Ministry of Transport says that driving too fast for the conditions is a only even one of the contributing factors in 29% of fatal crashes, so getting a 25% reduction in deaths just from tightening the tolerance seems beyond belief.  To be fair, the Minister only said the policy “contributed” to the reduction, so even one death prevented would technically count, but that’s not the impression being given.

What’s a bit depressing is that none of the media discussion I’ve seen of the summer campaign has asked what tolerance is actually needed, based on accuracy of speedometers and police speed measurements. And while stories mention that the summer campaign is a trial run to be continued if it is successful, no-one seems to have asked what the evaluation criteria will be and whether they make sense.

(suggested by Nick Iversen)


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

    This quantitative analysis of the 2011 road toll commissioned by the Ministry of Transport makes for an interesting read.


    I am not sure if this has been reported upon yet. It lists few factors which the analysis found to be statistically significant, vehicle speed is not one of them. I thought this is pertinent to the above discussion since it looks at the whole year’s data.

    3 years ago Reply

    • avatar
      Thomas Lumley

      Thanks, that’s a good source. The police and transport people on the end-of-year news summary stories seem to be using that script, but it gets forgotten when they talk about the anti-speeding campaign.

      3 years ago Reply

  • avatar

    And not just speedometer accuracy – think of foot accuracy among those of us without cruise control. With a 10kph tolerance, I can target 100 kph with only trivial chance of accidentally exceeding 110 kph. With a 4 kph tolerance, I cross 104 accidentally very frequently where there are hills. And so I either have to target 95 kph, or use the radar detector.

    3 years ago Reply

    • avatar
      Thomas Lumley

      It’s less clear to me that foot accuracy has to be included.

      Based on general principles of justice I think there’s a very strong case that keeping the speedometer needle below 100km/h should protect you from legal penalty, and the tolerance needs to ensure this.

      It’s not obvious to me that there’s a natural right to target 100 km/h rather than 95km/h. I’d characterize that as a policy question that ideally should be determined by public consultation.

      3 years ago Reply

  • avatar
    Nick Iversen

    A scientific approach would be to measure the speed camera readings of a sample of cars travelling at 100km/h as indicated on the speedo and using a sample of speed cameras. Then use the 99.9% percentile as the tolerance.

    My guess is that the tolerance will be 2 km/h since speedos are required to never read lower than the actual speeds and speed cameras have a margin of error of up to 1.8 km/h.

    The problem with that approach is that motorist will reason that they can drive at 102 knowing that the actual speed will be no more than that but that doesn’t take into account the camera accuracy because some cameras will indicate 104.

    So a tolerance of 102 means you shouldn’t drive faster than 100 on the speedo. Right now the tolerance of 104 means that you shouldn’t exceed 102. So Eric – you need to not exceed 102, not 104 when going down hills.

    3 years ago Reply

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