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Why do we always find ourselves in the slower lane?

March 3, 2018

Is the line next to you at the check-in at the airport or the check-out at the supermarket really always quicker than the one you are in? Is the traffic in the neighbouring lane always moving a bit more quickly than your lane? Or does it just seem that way?

One explanation is to appeal to basic human psychology. For example, is it an illusion caused by us being more likely to glance over at the neighbouring lane when we are progressing forward slowly than quickly? Is it a consequence of the fact that we tend to look forwards rather than backwards, so vehicles that are overtaken become forgotten very quickly, whereas those that remain in front continue to torment us? Do we take more notice, or remember for longer the times we are passed than when we pass others? If this is the complete explanation, it seems we should passively accept our lot. On the other hand, perhaps we really are more often than not in the slower lane. If so, there may be a reason. Let me explain using an example.

How big is the smallest fish in the pond? You catch sixty fish, all of which are more than six inches long. Does this evidence add support to a hypothesis that all the fish in the pond are longer than six inches? Only if your net is able to catch fish smaller than six inches. What if the holes in the net allow smaller fish to pass through? This may be described as a selection effect, or an observation bias.

Apply the same principle to your place in the line or the lane.

To understand the effect in this context we need to ask, ‘For a randomly selected person, are the people or vehicles in the next line or lane actually moving faster?’

Well, one obvious reason why we might be in a slower lane is that there are more vehicles in it than in the neighbouring lane. This means that more of our time is spent in the slower lane. In particular, cars travelling at greater speeds are normally more spread out than slower cars, so that over a given stretch of road there are likely to be more cars in the slower lane, which means that more of the average driver’s time is spent in the slower lane or lanes. This is known as an observer selection effect, a key idea in the theory of which is that observers should reason as if they were a random sample from the set of all observers. In other words, when making observations of the speed of cars in the next lane, or the progress of the neighbouring line to the cashier, it is important to consider yourself as a random observer, and think about the implications of this for your observation.

To put it another way, if you are in a line and think of your present observation as a random sample from all the observations made by all relevant observers, then the probability is that your observation will be made from the perspective that most drivers have, which is the viewpoint of the slower moving queue, as that is where more observers are likely to be. It is because most observers are in the slower lane, therefore, that a typical or randomly selected driver will not only seem to be in the slower lane but actually will be in the slower lane. Let’s put it this way. If there are 20 in the slower lane and 10 in the equivalent section of the fast lane, there is a 2/3 chance that you are in the slow lane.

So the next time you think that the other lane is faster, be aware that it very probably is.



Nick Bostrom, Anthropic Bias: Observation Selection Effects in Science and Philosophy, Chapter 1, Routledge, 2002.

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