Defective colour vision is a risk factor in driving

Abstract

Verriest et al (1980) claimed ‘definite proof that colour-defective drivers do not have more accidents than people with normal colour vision’ by showing that colour-defective drivers were not over-represented in a sample of 2058 male drivers who had caused a traffic accident. In the same paper, however, it is shown that protans have significantly more rear-end collisions and other accidents caused by overlooking signal lights. Deutans had more accidents at traffic lights. The differing conclusions can only be reconciled by assuming that colour-defective drivers have fewer other accidents to compensate for their greater frequency of accidents involving signal lights, and this is inherently improbable.

Verriest et al were incorrect to conclude, from their data, that defective colour vision is not a risk factor in driving, because their sample size was too small and the study lacked a control group. Even so, they found that 8.41% of the accident-causing sample had defective colour vision, somewhat more than the usually-assumed 8%.

The odds ratio for defective colour vision as a risk factor, calculated from comparison of the Verriest et al data with those reported by European investigators, was 1.11 This is not significant, but the confidence interval raises the statistical possibility that the odds ratio may be as high as 1.31.

It is to be expected that drivers with defective colour vision will have difficulty locating and recognizing traffic signals because of their reduced ability to differentiate colours and, in the case of protans, their reduced ability to see red signals. We present data that shows that about 20% of anomalous trichromats and nearly 60% of dichromats admit to difficulty recognizing colours while 10-15% of protans admit to difficulty seeing red signal lights. The accident data of Verriest et al (1980) and also those of Hager (1963), taken with other evidence, lead to the conclusion that defective colour vision is a risk factor for driving. However, it is a risk that bears only on the 8% of drivers who have defective colour vision and the number of accidents due to defective colour vision will therefore be relatively small, probably no more than 4 in 1000 accidents. Nevertheless it would be in the public interest to apply a colour vision standard at least for drivers of commercial and public transport vehicles, since they drive greater distances than private drivers and the social cost of accidents involving commercial and public transport vehicles is higher.