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Road safety study Speeding more likely when driving in low light

Time exposure shot of a Swiss highway at night

Full speed ahead: it seems the dark, as well as the light, can play tricks on our perception.


You might expect that a dark road ahead would prompt motorists to slow down, but a collaborative study led by researchers at the University of St Gallen has reached the opposite conclusion.

The researchers found that motorists were significantly more likely to break the speed limit when the surroundings were dark, and to react to conditions of lower light by increasing their driving speed. They observed this correlation even when other factors like traffic and the time of day were taken into account.

The study, published in the open-access scientific journal PLOS Oneexternal link, is based on data gathered by tracking 1.2 million vehicle movements in the city of Zurich.

The researchers attribute their finding to a distortion in drivers’ perception of illuminance, which relates to the amount of light energy striking a surface: when the light is low, we tend to think we are moving more slowly than we really are, and to accelerate in response.

This distortion could be due in part to a phenomenon in which reduced contrast – or a smaller difference in brightness between an object and its background – can lead us to underestimate speed, the researchers say.

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Policy recommendations

The researchers hope their data could help inform road safety policy, as traffic accidents kill over a million people worldwide every year, and a third of those deaths can be attributed to excessive speed. They recommend that policy-makers educate motorists about this visual phenomenon, and consider implementing technologies such as smart street lighting and better vehicle headlights to help prevent speeding.

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