Why are experts not better in judging the danger of filmed traffic conflicts?

Earlier studies on the subject of subjective judgment of traffic conflicts showed that untrained subjects can reliably judge the dangerousness of filmed traffic conflicts. It was concluded that these judgements were based on a concept of dangerousness. In line with these findings two experiments were conducted. Experiment 1 tested the hypothesis that traffic experts (traffic engineers trained to evaluate and improve traffic safety) and lay people use the same concept of dangerousness when judging filmed conflicts. Experiment 2 investigated which aspects of the conflicts are considered by experts when making these judgements. The results show that (i) experts and lay people are equally reliable in judging traffic conflicts, (ii) experts base their judgement on the same concept of dangerousness, (iii) experts do not base their judgement on aspects that they themselves regard as important for the conflict, (iv) a weak relation is found between violations of traffic regulations and judgements of dangerousness.