Smeed's Law


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. The law proposes that increasing traffic volume (an increase in motor vehicle registrations) leads to an increase in fatalities per capita, but a decrease in fatalities per vehicle.

Smeed also predicted that the average speed of traffic in central London would always be nine miles per hour, because that is the minimum speed that people tolerate. He predicted that any intervention intended to speed traffic would only lead to more people driving at this “tolerable” speed unless there were any other disincentives against doing so.

His hypothesis in relation to road traffic safety has been disputed by several authors, who point 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.

Related formulas


D annual road deaths (dimensionless)
nnumber of registered vehicles (dimensionless)
ppopulation (dimensionless)