The technical performance of seat-belts is not in doubt, but their continuing value is diminished if driving deteriorates following the switch to seat-belt use. Janssen (1994) demonstrated such behavioral adaptation over one year after the switch, but the effects were not sufficient to nullify seat-belt use. Longer experiments are probably impractical because of near-universal seat-belt use. Longer experiments are probably impractical because of near-universal seat-belt legislation. However, UK statistics suggest that driving speeds have increased over the years since legislation to the extent of more than nullifying the effectiveness of seat-belts. In the present paper, fuller's learning model of road behavior is developed in conjunction with the perceptual phenomenon of looming as an alternative to risk formulations of behavioral adaptation. Looming acts as negative reinforcement for unbelted drivers, but not for belted drivers. Because it represents threat to life, negative reinforcement persists in its effectiveness: it will take some years for the loss of looming to affect fully the new seat-belt user's behavior. In addition, behavioral adaptation in motorists inevitably militates against the encouragement of environmentally-sustainable - but vulnerable - modes of travel, such as walking and cycling. If safety-related interventions are to be properly assessed, there must be adequate empirical data-obtained over a time-span which permits a full assessment of behavioral adaptation - in conjunction ith a plausible theoretical framework in which the data can be interpreted.
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