How safety measures can change drivers' behavior

By: Drivers.com staff

Date: Friday, 23. March 2007

This is a summary of an article by Stein Fosser, Fridulv Sagberg, and Inger Anne F. Stætermo, titled "Behavioral adaptation to airbags and antilock brakes", which was published in Nordic Road & Transport Research, No. 3, 1996.

Measures to improve safety don't always have the intended effect. Sometimes measures such as driving skills programs, antilock braking systems, airbags, enforcement programs produce changes in behavior that reduce the effects of the measures or counteract them entirely.

The behavior adaptation that follows such measures is often termed "risk compensation" and it can partly or completely offset the intended safety effects of measures. Studded tires, for example, give better traction on slippery surfaces, but they have been shown to be associated with increased driving speeds. It's likely that they also result in increased exposure to risk, since drivers may venture forth in conditions that would deter them from driving if they were not equipped with studded tires.

A study conducted by the Norwegian Institute of Transport Economics looked at behavioral adaptation to antilock braking systems (ABS) and airbags. The hypothesis was that accident-reducing measures such as ABS are compensated to a larger extent than injury-reducing measures such as airbags.

It seemed less likely that the injury-reducing measure (such as an airbag) would result in an increased risk of accident. A measure that reduces the risk of an accident, on the other hand, seems more likely to result in behavior that would tend to maintain accident risk.

An important factor is whether the individual is aware of the safety measure. Even more important is whether the measure produces feedback. For example, in the case of airbags the device produces no feedback whatsoever (except in the rare case of an accident). ABS, on the other hand, may give feedback on slippery roads in terms of steering performance and braking. This may give drivers a feeling of extra security.

Risk compensation may also be manifested in strategic driving decisions such as whether or not to make a trip or decisions about when and where to drive. It may also result in reduced use of safety equipment, for instance if airbags result in seat belts not being used.

The Oslo taxi driver study

In May, 1995, researchers from the Institute of Transport Economics studied the behavior of taxi drivers driving from Oslo city to the airport on a stretch of the E18 motorway.

Taxi drivers were chosen because this made it easier to randomly select a group. The possibility of individuals "self selecting" for the study was eliminated. In addition, since many taxi drivers don't own the taxi they are driving and are assigned to a vehicle rather than choosing it, the possibility that the presence or absence or ABS would be due to driver preference or personality was avoided. This insured that the equipment of the taxis was independent of driver characteristics.

By choosing the motorway from the city to the airport researchers were able to track the taxis to their destination and then use a questionnaire to get information about whether the cars were equipped with airbags or ABS.

The data basis for the behavioral measurements consisted of video-recordings of traffic on a straight 180-metre road section. Driver behavior was recorded for 1,384 different taxis looking at driving features such as speed, headway, lane changes, lane occupancy, and the variability of lateral position.

The questionnaire

Behavioral data from observation was matched with information from questionnaires administered when the taxis arrived at their airport destination. Drivers reported personal background information and answered questions about driving behavior. The questionnaire was completed by 517 drivers. Some analyses were based on the whole sample of drivers with questionnaire data, while the main analyses included only those cars for which a match was possible between questionnaire and driving data.

Drivers of taxis with both airbags and ABS reported less opportunity to drive as fast as they wanted to, compared with drivers who didn't have these equipments. In other words, there was a tendency for behavioral adaptation.

Separate comparisons for ABS and airbags showed that this difference was significant only for airbags. The tendency was not reflected in higher speeds, probably because the density of traffic prevented drivers from choosing speed at their own discretion.

Taxis with ABS had significantly shorter time headways than taxis without ABS. Simple comparisons showed a lower rate of seat belt use among drivers with ABS and fewer lane changes. However, multiple regression analysis indicated that the lower seat belt use might be explained by driver background factors or by car characteristics other than ABS or airbags.

The results of the study were unclear on the question of whether accident-reducing measures (such as ABS) might have a greater effect than injury-reducing ones (such as airbags). The headway results support the hypothesis, but self-reported data suggest possible compensation also for airbags.

More weight can be placed on the behavioral results since the self-reported data may be questioned on general methodological grounds.

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Sandra,

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