Seat belts: another look at the data
November 5th, 2009 by johnadams
I am grateful for a question posted today by Carsten Jasner in response to an earlier post of mine - Seat belts again. It has prompted another look at the data:
“Very interesting! But when the number of car occupant deaths increases while the number of all road user deaths decreases – how can the number of pedestrians and cyclists [deaths] also increase?”
Figure 1, all road accident deaths, shows that a well-established downward trend was interrupted (by the seat belt law?) and replaced by a slightly rising plateau. After the seat belt law (arrow) total deaths did not fall below the 1983 level until 1991.
Figure 1
For many decades, as car ownership increased in Britain the number of people moving about in cars also increased while the numbers moving about on feet or bicycles, and exposed to the risk of road accidents, fell sharply. Part of the decline in walking and cycling can be explained by the shift to car travel; another part by the fact, that as the volume of metal in motion increased, children were withdrawn from the threat, while vulnerable adults, especially cyclists, withdrew themselves. Figure 2 shows a dramatic decline since 1930 in the ratio of pedestrians and cyclists killed to car occupants killed - from 5.95 in 1935 to 0.47 in 2006.
Figure 2
Figure 3 zooms in on more recent years. Between 1970 and 1982 the ratio dropped from 0.96 to 0.81. In 1983, the first year of the seat belt law, the ratio jumped sharply to 1.00, before resuming its historic downward trend, but it did not drop below 0.81 until 1989. This sharp jump is of course exactly what one would expect in the light of the decrease in car occupant deaths and increase in pedestrian and cycling deaths coinciding with the seat belt law noted earlier. The step change in the trend suggests that each year since 1983 the seat belt law continues to deserve credit for the deaths of vulnerable road users, who but for the law would still be with us.
Figure 3
Thanks for your reply! There is something I haven´t understood yet: the historic downward trend of all road accident fatalities since the 1970s (from 7,000 to 2,500). Though more and more people have been moving around in cars. What causes this trend? I can imagine that parents withdraw their children from the streets. But that can´t be the only explanation.
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Carsten
Ruben Smeed first observed this phenomenon in an article in 1949. He found it to be universal. Every country for which he could find data experienced a decrease in deaths per vehicle as vehicles per capita increased. (See my discussion of the Smeed Law in Risk and Freedom Chapters 2 and 7.) There are myriad adjustments to the growing threat of traffic. Anyone who has lived in a country at the early stages of motorization will be familiar with a very different driving style. Car owners tend to be rich and powerful and drive with disdain for the chickens and pigs and peasants in their path. If you live in a village with little traffic you do not spend a lot of time drilling your children on the Green Cross Code. The withdrawl of children is a significant part of the explanation. In 1922 in Britain there was very little traffic and a nation-wide 20mph speed limit - and there were more than three times as many children killed in road accidents than today. The Smeed Curve might be described as a social learning curve.
I think the “withdrawal of children” as you put it is indeed a large part of the reason. There are simply fewer vulnerable road users on the streets of the UK for cars to crash into.
This is one of the things which is very different about the Netherlands. Dutch roads are overall the safest in the world and they are so despite a much larger percentage of vulnerable road users.
However, over here it is still the case that it’s extremely difficult for drivers of cars to crash into cyclists. As you can see in this video taken near our home.
The Dutch car ownership rate was overtaken by the UK some years ago.