Another nudge by illusion is a reminder of the force of the law by using fake police officers. In Bangalore, life-size khaki-clad cardboard cut-outs of policemen are used to persuade drivers to behave. Similar tricks are used in China, the United States and some European countries. In Preston, a town in the UK, motorists hit the brakes after mistaking life-sized metal replicas for real police officers. Some towns have placed life-size cardboard cut-outs of police cars on bridges crossing highways.

Towing the line

But not all nudging needs to be that elaborate or even lifelike. Painting jagged or sharp lines on the road can frighten drivers enough to slow down, says Prasad. The lines appear haphazard, and the unfamiliarity forces drivers to slow down.

Chicago’s Department of Transportation had success with another simple illusion on a notorious hot-spot for crashes, a bend at Oak Street on Lake Shore Drive. City officials tried nearly everything: making lane markings clearer, putting up bigger warning signs, flashing lights at the side of the road. All in vain; drivers just kept crashing.

In the end the department painted white lines across the road – each line closer to the next as cars got nearer the curve. This perception of shrinking distances makes drivers think that they’re going faster than they really are. According to a study there were 36% fewer crashes in the six months after the lines were painted in September 2006 compared to the same six-month period the year before. Hansen believes this is one of the best psychological tricks to reduce speeding.

Novelty effect

Such measures tend to work better than road signs because, like most visual information, they speak to our brain at a subconscious level. “When we see a sign, if we see a sign at all – for we may have trained our brains to typically ignore them – we may question its applicability to us, or its general validity,” says Tom Vanderbilt, the author of Traffic, a book that explores how “perceptual countermeasures” can improve safety on the road. “But driving down a quite narrow street, or over a series of whizzing bars, you are less likely to think about it and more likely to just reflexively slow down,” he says.

And this works much better than visual illusions, he adds – because the danger is once you know something is fake, you may quickly start to ignore it. “As with all experimental treatments, there is the question of a novelty effect, of drivers eventually returning to their old behaviour once they've 'figured out' the trick,” says Vanderbilt.

Seven years ago in India, one inventive maker of car suspensions placed 3D stickers on the road that looked like fake potholes. The plan was to persuade drivers to buy this company’s suspension systems, suggesting that drivers would enjoy a smooth ride even on rough roads. Instead the stickers persuaded speeding drivers to slow down.

Similarly, in Philadelphia, Pennsylvania, road planners painted 3D illusions of speed bumps. But, as Hansen points out, “a widespread introduction of fake speed bumps may ultimately lead to hazardous driving behaviour even at real speed bumps".

So how far can road-safety nudging go? Hansen thinks a lot more. For one, the dividing line could be removed, because “it works as a subtle cue to speed up, as it marks one’s ‘safe territory’,” he says. And why focus only on the roads? Steering wheels could come with two red handprints, only visible when the driver’s hands are not on the wheel, to get people to drive with both hands. “We psychologically prefer things to be in their right place – and the same might work for steering wheels.”

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