Monday, May 25, 2015

Walking can be more dangerous than cycling

We are often told that flying is the safest way to travel, at least as far as the use of commercial airlines is concerned. In an early stand-up comedy routine, Shelley Berman noted: "Statistics prove that flying is the safest way to travel. I don't know how much consideration they've given to walking!" Well, actually, they have included walking.

Governments like to keep a track of these things, and the Department for Transport in Great Britain has released statistics on "Passenger casualty rates for different modes of travel" for 2003-2012. These modes include:
  • Air (passenger casualties in accidents involving UK registered airline aircraft)
  • Rail (passenger casualties involved in train accidents and accidents occurring through movement of railway vehicles)
  • Water (passenger casualties on UK registered merchant vessels)
  • Bus or coach (passenger casualties)
  • Car (driver and passenger casualties)
  • Van (driver and passenger casualties)
  • Motorcycle (driver and passenger casualties)
  • Pedal cycle
  • Pedestrian
The data are yearly averages for Great Britain from 2003-2012 inclusive, standardized as persons per billion passenger kilometres. The data are provided separately for the number of people killed, seriously injured, or slightly injured.

As usual, we can employ a phylogenetic network as a form of exploratory data analysis for these data. I first used the manhattan distance to calculate the similarity of the seven transportation modes for which there are complete data, followed by a Neighbor-net analysis to display the between-mode similarities as a phylogenetic network. So, modes that are closely connected in the network are similar to each other based on their accident figures across the ten years, and those that are further apart are progressively more different from each other.

The probability of incidents increases from right to left in the graph.

Some notable conclusions from the data are:
  • The probabilities of being killed, seriously injured or even slightly injured are all minuscule for air travel compared to anything else. This is a topic explored more thoroughly in an earlier blog post (A network analysis of airplane disasters).
  • You are much more likely to be injured in a bus than in a truck, but more likely to be killed in the truck than in the bus.
  • You are slightly more likely to be killed walking than cycling, but much more likely to be injured cycling.
  • A motorbike is the most effective way to get killed or seriously injured in Britain.

The walking versus cycling data are likely to surprise many people, but the average data across the 10 years are clear:

Pedal cycle
Seriously injured
Slightly injured

Danny Yee (Walking and cycling: relative risks) provides one explanation:
People who wouldn't even contemplate wearing special high-visability clothing or a helmet for a walk to the shops do so when cycling the same route.

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