Smeed's law
Encyclopedia
Smeed's Law, named after R. J. Smeed
, who first proposed the relationship in 1949, is an empirical rule relating traffic fatalities to traffic congestion
as measured by the proxy
of motor vehicle
registrations and country population. Thus, increasing traffic volume leads to an increase in fatalities per capita, but a decrease in fatalities per vehicle. It was posited after an analysis of figures from a number of countries over several decades.
Some researchers now dispute this, pointing out that fatalities per person have decreased, when the "Law" requires that they should increase as long as the number of vehicles per person continues to rise.
or, weighted per capita,
where D is annual road deaths, n is number of registered vehicles, and p is population.
Smeed published his research for twenty different countries, and, by his death in 1976, he had expanded this to 46 countries, all showing this result. Smeed became deputy director of the Road Research Laboratory
and, later, Professor at University College London
.
: people take advantage of improvements in automobiles or infrastructure to drive ever more recklessly in the interests of speed until deaths rise to a socially unacceptable level, at which point, safety becomes more important, and recklessness less tolerated.
Freeman Dyson
summarized his friend's view as:
Whilst in charge of the RRL's traffic and safety division, Smeed's views on speeds and accidents were well reported at the time of the introduction of a mandatory speed limit on UK roads: "If I wanted to stop all road accidents I would ban the car and introduce an overall speed limit, for there is no doubt that speed limits reduce accidents. Of course, roads with higher speeds often have lower accident rates. It is only on the safer, clear roads that you can drive fast - but that does not prove that you are driving more safely." He recognised that few methods of reducing accidents were painless and thus preferred to report facts and not to make direct recommendations as: "political, social and economic factors come in - but the people who make the decisions must know what the facts are on a subject.".
At the opposite end of this theory was Smeed's observations of heavily congested networks. He noted that at some minimum speed, motorists would simply choose not to drive. If speeds fell below 9 mph, then drivers would keep away; as speeds rose above this limit, it would draw more drivers out until the roads became congested again.
They generally suggest that fatality rates per vehicle are now decreasing faster than the formula would suggest, and that, in many cases, fatality rates per person are also falling, directly contrary to the Smeed's Law prediction. They attribute this improvement to effective safety interventions.
Powles (Oxford Textbook of Public Health) notes that the Australian state of Victoria which experienced deaths in excess of the Smeed formula until about 1970, subsequently adopted a range of interventions which took it from being a poor performer in terms of road safety to one of the best. Deaths fell in absolute terms from a peak of 1000 in 1970 to below 300 in 2009, despite strong growth in population and the number of vehicles.
However the relationship was revisited by John Adams
and disputed this revisionist thinking. He held that Smeed's law linking deaths, vehicle-miles and population was still valid for a variety of countries over time. In 1995, Adams claimed the relationship worked for the data of 62 countries. He noted an enormous difference in fatality rates across different parts of the world in spite of safety interventions, and suggested that Smeed's Law was still useful in establish general trends, especially when using a very long time period. Variations from the trend were normally better explained through economics, rather than claimed safety interventions. However, Adams found that Smeed's calculation of estimated deaths from vehicles per population was less successful than the calculation for vehicle-miles.
Reuben Smeed
Reuben Jacob Smeed was a British statistician and transport researcher.He obtained a degree in mathematics and PhD in aeronautical engineering from Queen Mary's College before entering academia as a teacher of mathematics....
, who first proposed the relationship in 1949, is an empirical rule relating traffic fatalities to traffic congestion
Traffic congestion
Traffic congestion is a condition on road networks that occurs as use increases, and is characterized by slower speeds, longer trip times, and increased vehicular queueing. The most common example is the physical use of roads by vehicles. When traffic demand is great enough that the interaction...
as measured by the proxy
Proxy (statistics)
In statistics, a proxy variable is something that is probably not in itself of any great interest, but from which a variable of interest can be obtained...
of motor vehicle
Motor vehicle
A motor vehicle or road vehicle is a self-propelled wheeled vehicle that does not operate on rails, such as trains or trolleys. The vehicle propulsion is provided by an engine or motor, usually by an internal combustion engine, or an electric motor, or some combination of the two, such as hybrid...
registrations and country population. Thus, increasing traffic volume leads to an increase in fatalities per capita, but a decrease in fatalities per vehicle. It was posited after an analysis of figures from a number of countries over several decades.
Some researchers now dispute this, pointing out that fatalities per person have decreased, when the "Law" requires that they should increase as long as the number of vehicles per person continues to rise.
Smeed's formula
Smeed's formula is expressed as:or, weighted per capita,
where D is annual road deaths, n is number of registered vehicles, and p is population.
Smeed published his research for twenty different countries, and, by his death in 1976, he had expanded this to 46 countries, all showing this result. Smeed became deputy director of the Road Research Laboratory
Transport Research Laboratory
TRL is a British transport consultancy and research organisation based at Wokingham Berkshire with approximately 500 staff. TRL is owned by the Transport Research Foundation , which is overseen by 80 sector members from the transport industry. TRL also own small UK regional offices situated in...
and, later, Professor at University College London
University College London
University College London is a public research university located in London, United Kingdom and the oldest and largest constituent college of the federal University of London...
.
Smeed's interpretation
Smeed claimed his law expresses a hypothesis of group psychologySocial psychology
Social psychology is the scientific study of how people's thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others. By this definition, scientific refers to the empirical method of investigation. The terms thoughts, feelings, and behaviors include all...
: people take advantage of improvements in automobiles or infrastructure to drive ever more recklessly in the interests of speed until deaths rise to a socially unacceptable level, at which point, safety becomes more important, and recklessness less tolerated.
Freeman Dyson
Freeman Dyson
Freeman John Dyson FRS is a British-born American theoretical physicist and mathematician, famous for his work in quantum field theory, solid-state physics, astronomy and nuclear engineering. Dyson is a member of the Board of Sponsors of the Bulletin of the Atomic Scientists...
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.
Whilst in charge of the RRL's traffic and safety division, Smeed's views on speeds and accidents were well reported at the time of the introduction of a mandatory speed limit on UK roads: "If I wanted to stop all road accidents I would ban the car and introduce an overall speed limit, for there is no doubt that speed limits reduce accidents. Of course, roads with higher speeds often have lower accident rates. It is only on the safer, clear roads that you can drive fast - but that does not prove that you are driving more safely." He recognised that few methods of reducing accidents were painless and thus preferred to report facts and not to make direct recommendations as: "political, social and economic factors come in - but the people who make the decisions must know what the facts are on a subject.".
At the opposite end of this theory was Smeed's observations of heavily congested networks. He noted that at some minimum speed, motorists would simply choose not to drive. If speeds fell below 9 mph, then drivers would keep away; as speeds rose above this limit, it would draw more drivers out until the roads became congested again.
Other research
The validity of Smeed's Law has been disputed by several other authors (Andreassen, Broughton, Oppe, and Ameen & Naji).They generally suggest that fatality rates per vehicle are now decreasing faster than the formula would suggest, and that, in many cases, fatality rates per person are also falling, directly contrary to the Smeed's Law prediction. They attribute this improvement to effective safety interventions.
Powles (Oxford Textbook of Public Health) notes that the Australian state of Victoria which experienced deaths in excess of the Smeed formula until about 1970, subsequently adopted a range of interventions which took it from being a poor performer in terms of road safety to one of the best. Deaths fell in absolute terms from a peak of 1000 in 1970 to below 300 in 2009, despite strong growth in population and the number of vehicles.
However the relationship was revisited by John Adams
John Adams (geographer)
Professor John Adams of University College London, is a professor of geography and leading theorist on risk compensation and an environmentalist...
and disputed this revisionist thinking. He held that Smeed's law linking deaths, vehicle-miles and population was still valid for a variety of countries over time. In 1995, Adams claimed the relationship worked for the data of 62 countries. He noted an enormous difference in fatality rates across different parts of the world in spite of safety interventions, and suggested that Smeed's Law was still useful in establish general trends, especially when using a very long time period. Variations from the trend were normally better explained through economics, rather than claimed safety interventions. However, Adams found that Smeed's calculation of estimated deaths from vehicles per population was less successful than the calculation for vehicle-miles.