November 22, 2012

Fly away home

With the summer holiday season approaching we’ve had requests for a post on the relative safety of driving and flying.

To a large extent this depends on where you are going: if you’re heading from Auckland to the Coromandel then I’d recommend driving, but if you want to spend some time on a beach in the Cook Islands your chances of getting there safely by car are distressingly low.

Clearly we need to rephrase the question.  Two possibilities are:

  • for a destination where either flying or driving makes sense, which one is safer?
  • if you compare a typical holiday road-trip to a typical holiday flight, which is safer?

We should also think about what risks to include: for a long plane flight the chance of a pulmonary embolism is higher than a crash, possibly much higher depending on your other risk factors.

The risk of a `fatal incident’ on a flight is largely independent of the length of the flight, and based on US data is about eight deaths per hundred million flights.  The risk is probably lower in NZ, since the figure includes the September 11 terrorist attacks.

The risk of death from car crash when driving in the US is about 4 per billion kilometers.  I don’t have good figures for NZ, but it’s a bit higher here. On the other hand, there’s a lot of variation depending on how you drive.

So, for a trip of 500km (eg, Auckland-Wellington), we’re looking at an average figure of about eight deaths in crashes per hundred million flights and about 200 deaths in crashes per hundred million car trips. Flying wins by a huge margin

University of Otago research estimates the risk of pulmonary embolism at about 0.5 per million short flights and about 1.3 per million long flights.  Estimates of the risk of death with pulmonary embolism in modern times seem to be around 10-20%, giving death rates of about 50-100 5-10 per hundred million short flights or 120-250 12-25 per hundred million long flights.  Flying still wins for the Auckland-Wellington route, even if driving doesn’t increase pulmonary embolism risk at all (it probably increases it but by less than driving)

If you compare a 500km drive with a long-haul intercontinental flight the numbers get less clear.  Flying to London could possibly be more dangerous than driving to Wellington, especially if you are a safe driver but at relatively high risk of blood clots.

After all these calculations it’s important to keep a sense of perspective. Driving is pretty safe. Flying is even safer.

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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

    Your arithmetic seems to be wrong on the embolism part:

    0.5 per million on short flights gives 50 per hundred million flights so with 10-20% dying, you get 5-10 deaths per hundred million flights. Another win for flying!

    2 years ago Reply

  • avatar
    DetMackey

    Need to adjust for the horror factor.

    2 years ago Reply

    • avatar
      Thomas Lumley

      It’s a fair point, but I’m not entirely convinced that I do need to adjust. You may need to.

      There are two different factors involved: people overestimate the probability of dying in a plane crash, because of various cognitive biases, and they may or may not be willing to accept a higher probability of death in a car crash than in a plane crash.

      The estimation bias is simply wrong, the different risk preference is a fact of human nature. In a sense you can distinguish them according to whether they go away when people are better informed.

      In this case I think the estimation bias is a bigger contributor. More importantly, it’s also the one where numbers are relevant.

      2 years ago Reply

    • avatar
      Thomas Lumley

      Though that paper doesn’t mention venous thromboembolism, which is by a large factor the largest risk in flying.

      8 months ago Reply

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