, risk compensation
is an effect whereby individual people 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. Another way of stating this is that individuals will behave less cautiously in situations where they feel "safer" or more protected.
There is evidence that suggests that such an effect is seen in humans
, associated with the use of safety features such as car seat belts
and bicycle helmets
. The evidence is particularly compelling for the case of antilock braking systems
. The existence of this balancing behaviour does not mean an intervention does not work; the effect could be less than, equal to, or even more than the true efficacy of the intervention, depending on how well the perceived efficacy matches actual efficacy - and this will differ from individual to individual. It is likely to be least when an intervention is imperceptible and greatest when an intervention is intrusive or conspicuous.
The theory grew largely out of investigations of road safety interventions. It was noted that most 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 more widely accepted, although it is still resisted by many who support an interventionist approach.
The logical conclusion of this theory has been reached with the shared space initiatives piloted first in Denmark and the Netherlands, and now being copied elsewhere in Europe and North America. Significant safety benefits have been claimed from the complete removal of street furniture and signage from urban environments, requiring all users to take more care.
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
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).
A study published in the March 2007 issue of Accident Analysis & Prevention
showed that drivers drove an average of 8.5 cm
closer, and came within 1 meter 23% more often, when a cyclist was wearing a helmet.
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
- published in 1982 by the Society of Automotive Engineers
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 a report into the issue. In the event the report's author, Isles, agreed with Adams' conclusions. The Isles Report was never published officially but a copy was leaked to the Press some years later. The law was duly passed and subsequent investigation showed some reduction in fatalities, the cause of which could not be conclusively stated, due to the simultaneous introduction of evidential breath testing.
Other research has taken groups of drivers, including those who did and did not habitually wear seat-belts, and measured the effect on driving style in the habitually unbelted. The drivers were found to drive faster and less carefully when belted.
Booth's rule #2, coined by skydiving pioneer Bill Booth
, states that "The safer skydiving gear becomes, the more chances skydivers will take, in order to keep the fatality rate constant". Even though skydiving equipment has made huge leaps forward in terms of reliability in the past two decades, and safety devices such as AADs
have been introduced, the fatality rate has stayed roughly constant since the early 1980s.
This can largely be attributed to an increase in the popularity of high performance canopies, which fly much faster than traditional parachutes. High speed manoeuvres close to the ground have increased the number of landing fatalities in recent years,
even though these jumpers have perfectly functioning parachutes over their heads.
A 1994 study by Jeremy Jackson and Roger Blackman, using a driving simulator, showed that although increased speed limits and reduced speeding fines significantly increased driving speed, there was no effect on accident frequency. It also showed that increased accident cost caused large and significant reductions in accident frequency but no change in speed choice. The abstract states that the results suggest that regulation of specific risky behaviors such as speed choice may have little influence on accident rates.
Safety equipment in children
At least two experimental studies have suggested that children who wear protective equipment are likely to take more risks.
An associated theory is known as risk homeostasis
. This extends risk compensation theory somewhat, although in practice the two terms are often used interchangeably.
Professor Gerald J. S. Wilde illustrates this by reference to the Swedish experience when they changed from left- to right-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.
Risk compensation is now widely accepted, but risk homeostasis, which goes much further, has a much smaller following.
The offset hypothesis
is the prediction that people respond to improvements in products meant to improve safety by acting less safely. A study led by Fred Mannering, a professor of civil engineering
at Purdue University
, suggests that the offset hypothesis is correct.