In the 1930s a traffic engineer in England noticed a curious pattern in the data about highway deaths.  Here is the chart from the article he published.

The vertical axis shows deaths/car and the horizontal shows cars/person with one dot for each country.  That’s for 1938.   In 1938 few people in Spain(19) owned a car, but those that did were causing a lot of deaths.   Switzerland(2) wasn’t fitting the model very well.   You can make up your own insta-theory for why countries with few cars/person kill more people with each car.

Here’s a chart from 1980.  More countries, more years, more confirmation of the model.  The data are shown twice, the second time is a log-log graph.

Note that there are lots of things you might think would affect the numbers for a given country.  For example: seat belts, population density, driver median age, safety regulations, insurance, policing, road quality, dash-board cams…  But those aren’t part of this simple curve and so can only effect the residuals.

I stole these charts from J.G.U. Adams short article “Smeed’s Law: some further thoughts” in Traffic Engineering and Control, 1987

I find this all weird.   You would think the traffic engineers would have a polished consensus by now of what this is saying.  Adams’ article has some interesting things to say.  For example societies learn to manage the cars as their numbers increase.   But I don’t sense there is a consensus in the profession.  Even now, 80+ years after the pattern was first noticed.