Many people (including about 8 to 10 percent of all males) inherit conditions in which either the “red” or the “green” photoreceptors do not transduce light properly. Such people have difficulty distinguishing hues that to typical individuals appear as red or green. Unfortunately, in Canada, traffic signals use red and green lights to indicate whether cars should stop or go through an intersection. Why do drivers with red–green colour blindness not risk auto accidents every time they approach an intersection?

AGE FOTOSTOCK/SUPERSTOCK