This question is an off-shoot of from the discussion about helmet safety. There is an assertion that when a car passes a bicycle, it allows less space to those who wear a helmet than those without. While this is metric is only one part of the larger picture regarding bicycle safety, it is often cited by helmet skeptics as scientific evidence to show wearing helmets has a negative effect on safety.
The assertion seems to base on a single study by Ian Walker. He says
"Drivers also tended to pass notably closer to the rider when he wore a helmet than when he did not."
And he reasons that
"The helmet effect is likely the result of drivers judging cyclists’ skill levels from their appearance and adjusting their overtaking accordingly"
I'm rather skeptical of his study. The chief problem is, in the eyes of a driver, bicycle helmets are not a salient feature. The driver's cognitive capacity is already taxed in judging the relative speed and space required for passing and often have to make a move in a short time window. There is little capacity to check out other details like helmets, let alone to use this to make a judgment and consciously vary his driving strategy.
Has anyone taken a critical look at his study? Does anyone have another related result?