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In ethology, risk compensation (sometimes known as risk homeostasis) is an effect whereby individual animals may tend to adjust their behaviour in response to perceived changes in risk. It is seen as self-evident that individuals will tend to behave in a more cautious manner if their perception of risk or danger increases. It is argued that the reverse also applies and that individuals may tend to behave less cautiously in situations where they feel "safer" or more protected.
There is evidence that such an effect is seen in humans, associated with the use of safety features such as car seat belts, antilock braking systems and bicycle helmets.
The theory grew largely out of investigations of road safety interventions. It was noted that interventions had failed to achieve the forecast savings in lives and injuries. Theorists speculated that while the studies demonstrated that the probability of injury given a crash had reduced, the fact that the overall probability of injury was unchanged indicated that there must have been some change in the probability of crashing.
This controversial view was at first strongly resisted but detailed investigation, particularly of the case of compulsory seat belts, caused the theory to become much more widely accepted.
Seat Belts
In 1981 John Adams published a paper, The efficacy of seatbelt legislation: A comparative study of road accident fatality statistics from 18 countries, Dept of Geography University College, London 1981. This showed that in the countries studied, which included states with and without seat belt laws, there was no correlation between the passing of seat belt legislation and reductions in injuries or fatalities.
This paper was published at a time when Britain was considering a seat belt law, so the Department of Transport commissioned the Isles Report into the issue, which it confidently predicted would contradict Adams' findings. In the event, Isles concluded that Adams was right. The Isles report was never published officially, a copy was leaked to the Press some years later. The law was passed, and there was no associated reduction in fatalities (although there was some reduction due to the simultaneous introduction of evidential breath testing). Although the number of drivers killed and injured decreased, there was a 75-year high increase in the numbers of pedestrians, cyclists and rear seat passengers killed.
Subsequent research has taken groups of drivers including those who did and did not habitually wear seat-belts and, under the guise of testing new types of belt material, measured the effect on driving style in the habitually unbelted. They were found to drive faster and less carefully when belted.
Antilock Brakes
There are at least three studies which show that drivers' response to antilock brakes is to drive faster, follow closer and brake later, accounting for the failure of ABS to result in any measurable improvement in road safety. The following references describe studies in Canada, Denmark and Germany.
Grant and Smiley, "Driver response to antilock brakes: a demonstration on behavioural adaptation" from Proceedings, Canadian Multidisciplinary Road Safety Conference VIII, June 14-16, Saskatchewan 1993
Sagberg, Fosser, and Saetermo, "An investigation of behavioural adaptation to airbags and antilock brakes among taxi drivers" Accident Analysis and Prevention #29 pp 293-302 1997
Aschenbrenner and Biehl, "Improved safety through improved technical measures? empirical studies regarding risk compensation processes in relation to anti-lock braking systems." In Trimpop and Wilde, Challenges to Accident Prevention: The issue of risk compensation behaviour (Groningen, NL, Styx Publications, 1994)
Cycle Helmets
The editor of Injury Prevention, a dedicated promoter of cycle helmets, long disputed the idea of risk compensation in cycle helmets, particularly in children. He set out to prove settle the issue; his study (http://www.pulsus.com/Paeds/09_05/mok_ed.htm), Risk compensation in childrens activities: A pilot study, D Mok, G Gore, B Hagel, E Mok, H Magdalinos, B Pless, Paediatrics & Child HealthMay/June 2004, Volume 9, Number 5: 327-330, showed strong evidence that children do indeed exhibit balancing behaviour.
Risk Homeostasis
An associated theory is known as risk homeostasis. This extends risk compensation theory somewhat, although in practice the two terms are often used interchangeably.
Wilde (http://ip.bmjjournals.com/cgi/content/full/4/2/89) illustrates this by reference to the Swedish experience when they changed from right- to left-hand drive in 1967. This was followed by a marked reduction in the traffic fatality rate, but the trend returned to its previous values after about 18 months. Drivers had responded to increased perceived danger by taking more care; as they became accustomed to the new regime, the additional care evaporated.
Further Reading
Risk, John Adams, Routledge, 1995, ISBN 1857280687
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