Road safety/Risk compensation

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The concept of risk compensation is well understood. Some of the studies which underly the theory bear some study.

The Munich taxicab experiment

The Munich taxi driver experiment[1] is one of a handful of experiments to systematically examine the effect of a safety intervention on driver behaviour.

Part of a taxi fleet in Munich was equipped with an Anti-lock braking system (ABS). ABS brakes are an example of a change in "intrinsic risk" — a change in the objective accident loss expected if drivers don't change the balance of their driving behaviour when a safety intervention is made available. Risk compensation theory would expect drivers to change their behaviour in response to this, and a measure of that would be to compare their accident likelihood per hour of driving. Wilde's theory of risk homeostasis posits that the level of risk will remain the same, but this is a subset of risk compensation theory and for it to be the case would require a very accurate understanding of the efficacy of ABS on the part of the drivers.

The cars with and without ABS in that Munich taxi fleet were of the same make and identical in all other respects. The majority of cab drivers were randomly assigned to one or the other of the two types of cars and the remaining drivers rotated between driving one type or the other. The exposure to traffic of each of the ABS taxicabs was carefully matched with cabs with traditional brakes over a period that lasted three years. Due to the matching procedure there was no difference in the time of day, the day of the week, the seasons, and the weather conditions in which both types of cabs were in operation.

Among a total of 747 accidents incurred by the company's taxis during that period, the involvement rate of the ABS vehicles was not lower, but slightly higher, although not significantly so in a statistical sense. These vehicles were somewhat under-represented in the sub-category of accidents in which the cab driver was judged to be culpable, but clearly over-represented in accidents in which the driver was not at fault. Accident severity was independent of the presence or absence of ABS.

In another part of their investigation, the researchers installed accelerometers in ten ABS and ten non-ABS cars, without the drivers' knowledge. These sensors measured the g-force of acceleration and deceleration once every ten milliseconds for a total of 3276 hours of driving. It was found that extreme deceleration, that is, extremely hard braking, occurred more often in the vehicles with ABS.

A third part of the study consisted of observations of driving style. Observers were trained in the systematic observation of a person's driving style and in recording their evaluations on rating scales. They were then instructed to call a taxi and to observe the traffic manners of the driver while they were passengers. A total of 113 such trips were made, 57 in cabs with ABS and 56 in cabs without. All trips covered the same 18 km route. Speed measurements were taken at four predetermined points of this route.

The drivers were not aware that their driver behaviour was being observed and the observers did not know whether they were in a taxi with ABS or without. The drivers did, of course, know whether or not they were operating an ABS cab, because of their familiarity with the car they were driving.

Subsequent analysis of the rating scales showed that drivers of cabs with ABS made sharper turns in curves, were less accurate in their lane-holding behaviour, proceeded at a shorter forward sight distance, made more poorly adjusted merging manoeuvres and created more "traffic conflicts". This is a technical term for a situation in which one or more traffic participants have to take swift action to avoid a collision with another road user.[2] Finally, as compared with the non-ABS cabs, the ABS cabs were driven faster at one of the four measuring points along the route. All these differences were significant.

In a further extension of their study, the researchers analysed the accidents recorded by the same taxi company during an additional year. No difference in accident or severity rate between ABS and non-ABS vehicles was observed, but ABS taxis had more accidents under slippery driving conditions than the comparison vehicles. A major drop, however, in the overall accident rate occurred in the fourth year as compared with the earlier three-year period. The researchers attributed this to the fact that the taxi company, in an effort to reduce the accident rate, had made the drivers responsible for paying part of the costs of vehicle repairs, and threatened them with dismissal if they accumulated a particularly bad accident record.

To sum up, in response to the installation of ABS brakes, drivers changed their driver behaviour. First, they utilized ABS to their advantage, but no improvement in the accident loss per time unit of exposure to traffic could be seen. Second, regardless of whether they were driving with or without ABS, a reduction in the accident rate did occur when the drivers' target level of risk was reduced by increasing their expected cost of risky behaviour.

The Munich taxicab experiment attracted a great deal of attention, not only in the professional circles, but also in the popular press. Newspapers carried articles about it and Bavarian Television wanted to show the viewers what had happened. As the experiment had already been completed, they decided to re-enact the experimental manipulation and the way the drivers had responded. Airing of this documentary added further to the popular debate. The results of this experiment were also discussed by a group of international experts from the Organisation for Economic Co-operation and Development, commonly abbreviated as OECD. In their final report, these experts from sixteen different countries stated that: "Behavioural adaptations of road users which may occur following the introduction of safety measures in the transport system are of particular concern to road authorities, regulatory bodies and motor vehicle manufacturers, particularly in cases where such adaptations may decrease the expected safety benefit."[3]

Another federal government wanted confirmation of the idea that drivers show an adaptation effect in response to ABS. The Canadian Ministry of Transport asked 81 drivers, selected from the general population, to perform a set of tasks while driving a car equipped with ABS which could be turned on or off at the turn of a switch. These tasks were carried out at the Transport Canada Test Centre in Blainville, Québec. They involved braking at a stop sign, accelerating to 70 km/h, emergency braking in a straight line, curve following, and emergency stopping in a curve. There was no interaction with other traffic on the test track. All drivers were informed of the features of ABS and told when they were driving with ABS on and when they would have to rely on standard brakes. Some drivers were given an opportunity to practise hard braking with ABS, while others were not.[4]

The most interesting results include the finding that, with the use of ABS, driving speeds and pressure exerted on the brake pedal were higher when drivers knew they were driving with the ABS system turned on. Further, higher maximum speed was observed in drivers who had experienced emergency braking with ABS as compared with those who had not. Most important, however, was the observation that the stopping distances during the braking manoeuvres were not any shorter in the presence of ABS than with standard brakes. They would have been shorter had the only driver response been to brake harder, without an increase in speed. Thus, the potential occurrence of shorter braking distances did not materialize. It was lost due to the fact that drivers utilized the more sophisticated brakes for higher speeds and harder braking, not for greater safety. This supports another statement in the above-mentioned OECD report: "An important conclusion of the Scientific Expert Group is that behavioural adaptation exists, and does have an effect on the safety benefits achieved through road safety programmes."[5]

Antilock brakes are a recent addition to a long string of supposed safety measures and they have been welcomed by many with great fanfare and heralded as a true life-saving device. It is interesting to note, however, that the limitations of better brakes in providing greater safety were already suspected in a footnote to a paper published almost 60 years ago, but the authors failed to identify motivational factors as the dominant determinants of the accident rate[6]: "...more efficient brakes on an automobile will not in themselves make driving the automobile any safer. Better brakes will reduce the absolute size of the minimum stopping zone, it is true, but the driver soon learns this new zone and...." You will have no difficulty in guessing the gist of the remainder of the sentence, but will you be able to resist the temptation of believing in the safety benefits promised by the next technological innovation, the next "technological fix", so to speak? Or will you still be waiting for Godot and willing to believe in the "unprecedented safety" heralded by the "high-tech" developments that are supposed to result in an "intelligent vehicle and highway system"?[7] By "safety benefits", we mean, of course, more safety per head of population, not per kilometre driven.

More difficult to understand are the first few words of the cited sentence, the first part that I left out, "Except for emergencies", and these words just do not seem to make sense. Maybe the authors were not quite certain that better brakes would fail to add to safety. What is certain is that the authors did not emphasize motivation or risk acceptance as the main determinant of safety. The quote comes from a mere footnote in their publication which, in general, emphasizes the importance of skill instead: "Safe and efficient driving is a matter of living up to the psychological laws of locomotion in a spatial field." As an aside, we may also note that psychological laws are supposed to describe the behaviour of any human being--including your behaviour and mine--whether we like these laws or not; we have no choice. Legal laws are different: we have the choice between compliance and going against them, and whether we do or not depends on psychological laws.

More safety per kilometre driven may well be expected from antilock brakes. Under dry pavement conditions, these brakes offer greater braking opportunity than standard brakes, because they are more likely to be activated with maximum foot pressure. Under slippery conditions, they offer the advantage of being able to brake and change direction at a lesser risk of skidding. Therefore, under both types of conditions, they offer an opportunity for greater speed without adding to unsafety. Driving faster means being able to drive a greater distance in the same amount of time. Drivers will thus have an opportunity for more mobility per time unit of driving. If greater mobility is attractive to them, they will drive more, with the end result that the accident loss per head of population does not change, while the accident rate per km driven is favourably affected. There is progress in the sense that people are given the opportunity of driving more kilometres per road accident. But at the same time there is stagnation in the sense that there's no reduction in the accident rate per person (see Equation 3 in Table 5.1).

Source: Target Risk, ch. 7.


  1. Aschenbrenner, M. and Biehl, B. (1994). Improved safety through improved technical measures? Empirical studies regarding risk compensation processes in relation to anti-lock braking systems. In R. M. Trimpop and G.J.S. Wilde, Challenges to accident prevention: The issue of risk compensation behaviour. Groningen, the Netherlands: Styx Publications.
  2. Hauer, E. and Garder, P. (1986). Research into the validity of the traffic conflicts technique. Accident Analysis and Prevention, 18, 471-481.
  3. OECD (1990). Behavioural adaptations to changes in the road transport system. Paris: Organization for Economic Co-operation and Development; Road Transport Research, p. 5.
  4. Grant, B.A. and Smiley, A (1993). Driver response to antilock brakes: a demonstration of behavioural adaptation. Proceedings, Canadian Multidisciplinary Road Safety Conference VIII, Saskatoon, Saskatchewan, June 14-16.
  5. OECD (1990). Behavioural adaptations to changes in the road transport system. Paris: Organization for Economic Co-operation and Development; Road Transport Research, p. 6.
  6. Gibson, J.J. and Crooks, L.E. (1938). A theoretical field analysis of automobile-driving. The American Journal of Psychology, 11, 453-471.
  7. Wilde, G.J.S. (1990). Véhicule informatisé et sécurité routière. Recherche -Transports -Sécurité, No. 26, 29-36.
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