A lot of effort goes into convincing motorists not to speed, especially around holiday periods.

Mainly we try to show that speeding can have bad consequences, from the cost and inconvenience of getting ticketed all the way through to injury and death.

But there is another reason not to speed that doesn't get much coverage, and that should convince everyone that once they are travelling above rather low speeds, accelerating even more doesn't make much sense.

How can that be? It turns out that the assumption that driving faster will get us where we are going quicker is flawed, and not just because of the standard concerns about increased risks of an accident. A 10km trip at 10km/h would take an hour. The same trip at 20km/h would take half an hour, a quite dramatic time saving. If we are interested in saving time - in getting where we are going more quickly - perhaps it makes sense to travel at 20km/h rather than 10km/h if we can. Suppose we speed up to 30km/h? Now our 10km trip will take 20 minutes. Travelling at 30km/h means we get where are going 10 minutes earlier than travelling at 20km/h: perhaps that will seem worthwhile too. Well, suppose we accelerate to 40km/h? Now our trip takes 15 minutes. Travelling at 40km/h rather than 30km/h means we get where are going five minutes earlier.

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Notice what is happening here. Speeding up does save us time, but less and less as we increase from higher starting speeds. The trend is constant, and by the time we get to the speeds we are likely to be interested in, the time savings are tiny. It turns out that the common, almost universal, assumption that we will get where we are going more quickly if we travel at higher speeds is, if not false, at least much less significant. Why do we persist in thinking travelling at higher speeds will make a significant difference to journey times? The tendency to overestimate the time saved by doing tasks more quickly is called the "time-saving bias", and it is one of a cluster of common reasoning errors all humans are inclined to make. Even people who are perfectly okay with basic maths tend to judge things like probability and how much time we save if we travel faster on the basis of "first impressions" rather than calculation.

We tend to judge the likelihood of something happening, for instance, by seeing how readily an example of that thing comes to mind, by how "available" it is to us. Availability may be a pretty good guide to probability: more common things should come to mind more readily than rare things. But things readily come to mind not only because they are common but because they are striking, because we have just seen an example on the news, or because we have some particular reason to care about that thing. People think plane crashes and terrible sporting accidents are more likely following a high profile example, and parents overestimate the probability of side-effects from vaccinations because they have an understandable concern about them: they come to mind more readily than they should given their actual probability. There is often a good reason for using these "heuristics", as they're called. They are quick and fairly reliable and - assuming many of them have their roots in the early evolution of our cognitive capacities - they probably worked in the environments in which we evolved. Being able to readily bring to mind food when we contemplated an environment might have been a pretty good way of estimating the probability of finding it there. But it is clear that these "quick and dirty" reasonings can lead us astray.

Obviously it's good to avoid these common reasoning errors. It helps us develop true beliefs and reject false ones. But it also has practical benefits. In the driving case, it should lead us to see that driving faster delivers almost no benefits. Once we see that we should feel less frustration when meeting slower drivers, and reduce the risks of accidents.

- Associate Professor Tim Dare is the head of Philosophy at the University of Auckland

- Views expressed here are the writer's opinion and not the newspaper's. Email: editor@hbtoday.co.nz