June 8, 2014

Foreign drivers

From the ChCh Press

Foreign drivers cause more fatal and injury crashes in the South Island than the national average – and the West Coast is the worst spot.

They don’t actually mean “more,” they mean “a higher proportion of”.

New Zealand Transport Agency (NZTA) safety directions chief adviser Lisa Rossiter said its crash statistics for the past 10 years showed foreign drivers were involved in about 6 per cent of all fatal or injury crashes in New Zealand, and were at fault in about 2 per cent.

On average, short-term visitors make up roughly 2.5% of people in New Zealand (2.78 million visitors in the year to April 2014, median visit of 9 days, so I’m guessing mean visit about two weeks). About another 2% of people in New Zealand are international students, who are at least sometimes counted as foreign drivers.

So, the risk seems to be a bit higher for foreign drivers, but probably not twice as high. Some of the excess can probably be explained by age: international students, backpackers, and drunk Australians in Queenstown are younger than the population average.

It’s different in parts of the South Island

The tourist hot spots of Otago and the West Coast fared worst.

A foreign driver was identified as a factor in 13 per cent of fatal crashes on the coast, and 5 per cent of fatal crashes in Otago from 2004 to 2013.

A lot of this must be because tourists are over-represented in tourist hot spots: that’s what ‘tourist hot-spot’ means. The proportion of short-term visitors is about 2.5% nationwide, but it’s probably rather lower that than in Gisborne and rather higher on the West Coast.

It’s also worth noting that “identified as a factor” is fairly weak. If you go to the Ministry of Transport reports and add up the percentage of times different factors were involved in a crash, you get a lot more than 100% (for the 2010 report I get 225% for fatal crashes and 185% for injury crashes)

For crashes involving a tourist driver and more than one car, the foreign driver was fully or partly responsible two out of three times.

This at least gets rid of the denominator problem, but the “partly” responsible is still a problem. We aren’t told what proportion of the time the local driver was fully or partly responsible — based on the information given, that could also be two out of three times.

It’s quite likely that foreign drivers are at higher risk, especially those from countries that drive on the right, but the problem is not a big fraction of the NZ road toll. It’s worth considering things that can sensibly be done to reduce it — which doesn’t include withdrawing from the U.N. Convention on Road Traffic — but if you’re trying to stop road deaths it may be more effective to concentrate on interventions that don’t just affect foreign drivers.  Clearer signage, guard rails and median barriers, separated bike lanes, improved public transport… there are many things that might knock a percentage point off road deaths more easily than targetting foreign drivers.

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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 »

Comments

  • avatar
    Helen Robinson

    It’d be interesting to know what percentage of drivers are overseas visitors. How many tourists don’t drive at all, but take tour busses, are on cruise ships, or stay in one city? This is probably more relevant than the number of visitors – if a relatively low percentage of them are driving, then the situation is probably worse than it looks (ie a higher percentage of drivers are crashing).

    3 months ago Reply

    • avatar

      On the other hand, tourists are much more mobile than residents – they tend to move around the country seeing stuff. So it’s also possible that the percentage of tourists on the road might be higher than the percentage they make up of the people in NZ on any one day.

      We at MBIE publish data on transport use by international tourists here: \http://www.med.govt.nz/sectors-industries/tourism/tourism-research-data/international-visitor-survey/online-database but I don’t think it would be able to answer the question with any precision. It suggests > 1m visitors a year using their own car or one owned by friends and family and >600,000 using hire cars (unfortunately can’t say how much overlap, from the published data).

      From that site I just linked to you can filter by country of residence and by purpose of visit, if you want to exclude Australians and VFR (Visiting Friends and Relatives) from your definition of ‘overseas’. But I leave this as an exercise…

      3 months ago Reply

    • avatar

      It’d be especially interesting to know what percentage of overseas visitors who choose to drive have never visited NZ before.

      Based on my experience from choosing not to drive in the US the first three times I went there, that cohort of drivers might be overrepresented.

      3 months ago Reply

      • avatar
        Thomas Lumley

        I don’t think this can be done from administrative data. You’d need to survey the car rental agencies to look at residence of customers. The information might be in an existing survey somewhere.

        3 months ago Reply

        • avatar
          Richard Penny

          And if you did it right you could even find out how many km each customer drove. The rental companies might even store this in their database (i.e. admin data, not survey data). Maybe not great data but better data than what we have.

          3 months ago

  • avatar

    Mean length of stay is actually about 20 days – see http://www.med.govt.nz/about-us/pdf-library/tourism-publications/KeyTourismStatistics.pdf. A bit longer than two weeks, which impacts a little on the calculations.

    3 months ago Reply

    • avatar
      Thomas Lumley

      Yes, the excess risk drops a bit. Thanks for the link.

      3 months ago Reply

  • avatar
    Richard Penny

    When I heard about this stuff my immediate thought was defining outcomes per “at risk” population – which is what statisticians do. My understanding of comparing traffic deaths between subpopulations is to say “at risk” is defined by per km driven, or number of trips as the former is harder to measure.

    I got this from a report an injuries for older drivers that pointed out that while they are fewer per head of population compared to other age groups, compared to number of trips they did – generally very few – it was not so good.

    Since I would expect tourists to, on average, drive longer distances than many NZers you could argue perhaps they are *less* dangerous.

    3 months ago Reply

    • avatar
      Thomas Lumley

      Yes, it’s hard to tell what subpopulations will be a problem. Young drivers are at higher risk, like old ones. And, of course, it’s even harder to get good data.

      I note than in the past I’ve compared regions on road deaths per capita, per unit distance and per registered vehicle. It makes relatively little difference, and the regions are not that dissimilar after standardisation.

      3 months ago Reply

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