Slow drivers 'thinking of dinner'
It's a phenomenon we've all experienced - drivers who cruise along at a leisurely pace on the open road, and then inexplicably speed up as soon as there's a passing lane, making it impossible to overtake without breaching the limit.
They exasperate many people, but road safety psychologist Dr Samuel Charlton said they almost certainly don't realise they're doing it.
"They're not doing it to slow people down or make them unhappy. They're just thinking about what they're having for dinner," he said.
"It is a frightening fact that drivers rarely give the task at hand their full attention. Rather we judge our speed subconsciously, particularly on long journeys."
Charlton said this explained sudden bursts of speed at passing lanes, which were generally placed in wide-open spaces.
"The brain uses information in the peripheral visual field to judge speed. For example when you go into a tunnel, most people slow down because you get increased peripheral visual stimulation, then you hit a passing lane, where there's nothing around you, and it seems to the eye that you're going slower," he said.
He said signs placed at the threshold of townships worked on the same principle. Drivers slowed down even when there was nothing on the signs because they had the sense of entering an enclosed area.
"The problem is this feeling wears out. We can't put signs all along the highway because we get used to it."
So what can be done?
"The best thing would be to look carefully at the locations of overtaking lanes," Charlton said.
"At the moment we put them wherever - flat and open spots usually. But that's the same time everyone gets the signal to speed up."