Smeed's law

From Wikipedia, the free encyclopedia

Smeed's Law, after RJ Smeed who first proposed the relationship in 1949, is an empirical rule relating traffic fatalities to motor vehicle registrations and country population. Thus annually increasing traffic volume leads to a decrease in accidents per vehicle. It was posited after an analysis of figures from a number of countries over several decades.

Smeed's formula is expressed as:

$D = .0003(np^2)^{1 \over 3}$

or, weighted per capita,

${D \over p} = .0003 \times {\sqrt[3]{n \over p}},$

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

Smeed showed this relationship worked with 20 different countries. The relationship was revisited by John Adams in 1987 who held that it was valid for a variety of countries over time, for example in Great Britain from 1909 to 1973. In 1995 Adams further showed the relationship worked for the data of 62 countries. However, the validity of Smeed's Law has also been disputed by several other authors (for example Andreassen 1985, Broughton 1988, Oppe 1991, Ammen & Naji 2000).

Smeed himself took his law as expressing a truth about human psychology: humans would take advantage of any improvements in automobiles or infrastructure to drive ever more recklessly in the interests of speed and safety until deaths rose to an unacceptable level, at which point they would cease to become more reckless; Freeman Dyson summarized his friend's view as:

Smeed had a fatalistic view of traffic flow. He said that the average speed of traffic in central London would always be nine miles per hour, because that is the minimum speed that people will tolerate. Intelligent use of traffic lights might increase the number of cars on the roads but would not increase their speed. As soon as the traffic flowed faster, more drivers would come to slow it down.....Smeed interpreted his law as a law of human nature. The number of deaths is determined mainly by psychological factors that are independent of material circumstances. People will drive recklessly until the number of deaths reaches the maximum they can tolerate. When the number exceeds that limit, they drive more carefully. Smeed's Law merely defines the number of deaths that we find psychologically tolerable.^[1]

[edit] References

^ See Dyson's "Part II: A Failure of Intelligence" in Tuesday, December 05, 2006, Technology Review

Smeed, RJ 1949. "Some statistical aspects of road safety research". Royal Statistical Society, Journal (A) CXII (Part I, series 4). 1-24.
Adams 1987. "Smeed's Law: some further thoughts." Traffic Engineering and Control (Feb) 70-73.
Adams 1995. Risk. London, UCL Press
Andreassen D. "Linking deaths with vehicles and population". Traffic Engineering and Control, November 1985.
Broughton, J. "Predictive models of road accident fatalities". Traffic Engineering and Control, May 1988.