Smeed’s Law – how well does it match experience in the UK?

Smeed’s Law – how well does it match experience in the UK?

Smeed’s Law is often cited by campaigns opposed to road safety measures, especially those who feel that they infringe personal freedoms. It suggests that the effect of such measures is cancelled out by changes in behaviour. If an activity is made less dangerous then people behave more dangerously because they feel safer, and the number of deaths stays the same.

The Law is expressed as a simple equation, with inputs of population and number of road vehicles. The output is a prediction of the number of road deaths per year.

I have never seen any of the campaigns that reference Smeed’s Law actually plot the data for the country in which they are campaigning. Out of interest, I plotted it for the UK:

The correlation starts off pretty well, but in the mid-1960′s it fails. Coincidentally this is when major road safety interventions started to be implemented in the UK.

This graphic is not embellished in any way. The red line is Smeed’s original 1949 equation fed with real UK official statistical data from 1951 to 2010; the blue line is UK official statistics on road deaths.

The equation

where D is deaths, n is number of vehicles registered, p is population.

Links

No need to trust my spreadsheet skills – here are the data sources so that you can check it yourself.

UK population data 1951 to 2001

Note – I interpolated for data points between census years, and used interim data for 2011 as census not yet published.

UK vehicle numbers data 1909 onwards

UK road traffic accidents data 1950 onwards

Wikipedia article

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