Opinion: It seems like such a uniquely New Zealand problem: You find yourself stuck behind a car that is crawling along as the road twists and turns its way through the countryside, only to have them speed up once you reach the passing lanes. Why does this happen?

One way to explain this phenomenon is to assume that the driver in the slower car is acting deliberately; that he or she is somehow trying to stop you overtaking them by accelerating ahead. And, in the process, that the other driver is consciously attempting to prevent you from reaching your destination in a timely manner. This view of other drivers sees the road as a place of contest and malice. A Darwinian struggle, red in tooth and claw, just to get to your destination. Explained like this, is it any wonder that people experience road rage?

Fortunately, there are better explanations we can draw on. As any good social scientist will point out, Hanlon's Law tells us that we should never attribute to malice that which is adequately explained by human frailty. The "frailty" in this case is one of perception, and in particular how our brains perceive speed. Simply put, narrower roads increase the perception of speed, and wider roads decrease that perception.

Which may seem obvious, but how does it explain why people actually speed up when the road widens? To do that, we need to refer to what is known as "risk homeostasis". This is the idea that all of us have a certain amount of perceived risk that we think is acceptable. When the perceived risk is below that particular level (or goes above it), we change our behaviour to adjust how much risk we feel. When a narrow road becomes wider (such as with the addition of a passing lane), the risk sensation decreases and our behaviour changes to reflect that.

Homeostasis works just like the thermostat in your heat pump at home, turning up the heat or cooling down the room to keep the desired temperature. You can see it in action in passing lanes as people speed up as the road widens and slow down as the passing lane ends and the road narrows. It may look like they are playing cat-and-mouse with you, but they're not (at least not most of the time).

Research from Europe demonstrates just how much impact road width can have on driving behaviour. Increasing the width of a road lane from 6m to 8m sees average speeds increase from 80kmh to between 90 and 100kmh. Moreover, adding to the number of lanes on a road (such as with passing lanes) produces faster speeds even where the width of individual lanes remains constant.

What is interesting about the link between road width and the perception of speed is that road designers clearly know this. They often use what are called "gateway treatments" to make roads appear narrower as they enter populated areas. These 'gateways' can be physical or they can simply be visual (such as different road markings).

Yet this understanding of how width affects the perception of speed seems strangely out of sync with the posters and signs that often get erected to remind drivers to be considerate, to pull over, and let others pass. That is, the built environment sends drivers one set of signals while the signs and posters attempt to send the opposite signal. In many ways that is like sitting down to the degustation menu at your favourite restaurant while surrounded by posters warning about the dangers of obesity.

Researchers also know that perceptions of speed are strongly influenced by peripheral vision and noise. The evidence is clear that peripheral vision deteriorates with age (with the size of our visual field decreasing by about three degree per decade). Researchers from the University of Chicago have argued that this leads to older drivers having lower risk thresholds (and hence driving slower) to compensate for this lack of vision.

Similarly, we all use noise to help estimate our speed. This means that better sealed roads (such as in passing lanes) will also lead to lower perceived speeds. Equally, it means that people in older cars may well think they are travelling faster than they are.

So why do people speed up in passing lanes? Because we have created the perfect environment to encourage them to do so. With the best will in the world, we have created a passing infrastructure that makes it difficult to pass.

This may seem like a cosmic joke but it is an example of what social scientists call "the law of unintended consequences". This warns us that interventions in complex systems tend to have unanticipated and often perverse outcomes. Which might point to the real insight contained in Hanlon's Law: that in the absence of proper understanding, human frailty often appears indistinguishable from malice.

Carl Davidson is the Head of Insight at Research First Ltd

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