Smeed’s Law, Seat Belts, and the Emperor’s New Clothes

Abstract

Smeed’s Law suggests that as motor vehicle ownership increases, death rates per vehicle decrease. The principal claims of the credit for this decrease are considered: safer vehicles, safer roads and safer road users.

The fact that Smeed’s Law applies both to individual countries over time, and to collections of countries at any given point in time, suggests that safer vehicles cannot be given any credit for the decrease in death rates over time; countries with low levels of vehicle ownership today are achieving death rates per vehicle, with modern imported “safe” vehicles, that are higher than those achieved with primitive vehicles in the early years of this century in Britain and the United States.

The explanation which appears to account best for the available data is Wilde’s risk homeostasis hypothesis: as traffic increases, attitudes toward risk and road user behavior both change in a way that maintains the levels of risk that individuals collectively are prepared to tolerate.

Claims made on behalf of vehicle safety regulations, accident black spot treatment and seat belt legislation are also examined and found wanting.

Seat belts are effective safety devices in accidents. But there is no convincing evidence that legislation compelling their use has reduced the total number of road accident deaths. The Wilde hypothesis suggests an explanation: protecting car occupants from the consequences of bad driving encourages bad driving.