Human error is a frequent contributor to road traffic collisions, understanding the human factors provides a deeper insight into road-user behaviour.

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Looked But Failed To See Errors

After an accident, people often report that they failed to see the other vehicle involved in the crash with enough time to avoid the collision, even though they looked in that direction. These “looked but failed to see” errors are very common and it has been estimated that not seeing another vehicle is implicated in up to 50% of collisions. Why are we so bad at detecting other vehicles, even when we actively look for them?

Look but fail to see errors can largely be explained in terms of the way in which humans process information. Our subjective view of the world feels rather like we are watching a film and that we smoothly look around the world with a perfect representation of the outside world being efficiently processed by our brain. Surprisingly, this is very far from the truth.

When we look at something, we actually only have clear vision in a small, central part of our visual field. If you hold you thumb up in front of you at arms length, our clear patch of vision is about the size of your thumbnail. Outside of this patch our vision becomes increasingly blurry. In addition to this, the way in which we move our eyes is not at all like the smooth camera we perceive. We tend to fixate our eyes on something for around a quarter to a third of a second and then move our eyes to a new location with this movement, or saccade, taking around 30-50 milliseconds. When we are moving our eyes we are, effectively, blind.

So, our visual input is actually very degraded. Each second we will see about three “snapshots” of our visual world and these snapshots are mainly quite blurry. In reality our vision is more like a cartoon flip book where a slightly different image is shown on each page, but flipping quickly through them gives the impression that the characters are moving. In addition to being blind when our eyes move, we also have no visual input when we blink. These two factors combined mean that we are, essentially, blind nearly 20% of the time! Additionally, research has shown that drivers at a junction may only look at the road that they are merging with for less than half a second[1].

Why does it seem like we have a clear and smooth moving perception of the world, when actually we only have blurry snapshots? Essentially our brains do some amazing “piecing together” of our snapshots and allow us to perceive a clear and smooth moving world. Part of this filling in is done on the basis of our experiences and existing knowledge. Imagine driving up to a busy junction near to where you live that you cross everyday. Your memory of this junction, called a schema, is part of what allows your brain to create a complete and clear image of it. Given this it’s easy to imagine how looked but failed to see errors can occur.

The limitations of our visual processing system can explain, at least to some extent, look but failed to see errors. But other factors also contribute. How conspicuous, or noticeable, a vehicle is also makes a big difference as to whether a driver sees it or not. Much of the research in this area has concentrated on motorcycle conspicuity given the relatively high frequency with which motorcycles are not seen before a collision. Given that the front of a motorcycle is relatively small, it seems quite obvious that it would be easier to miss, but what can be done to reduce this risk.

The use of headlights and wearing of bright clothing have been examined as two of the most likely ways of making motorcyclists more visible. The use of headlights during the daytime has been found to reduce the number of motorcycle accidents; however, interestingly this is also true for cars, trucks and buses[2]. It has also been suggested that headlight use is only increases conspicuity when a motorcycle is further away and that it is little use at shorter distances[3].

Research has also examined whether there are differences in looked but failed to see errors between novice and expert drivers. Typically these studies monitor where the driver is looking and then these patterns are compared between the two groups[4]. The experienced drivers had very different looking “strategies” when driving on a motorway, urban or rural road. This suggests that they vary their driving techniques according to the environment. In contrast, novice drivers had very similar patterns of eye movements and fixations on all three of the road types. One implication of the experienced drivers being able to adopt suitable driving behaviours for different road types, this may also come at a cost. Because of all their experience, these drivers are likely to have well established schemas of the roads they frequently drive on. Consequently, they may be less likely to see and oncoming vehicle if it does not “match” with their existing schema. Ironically, this means that experienced drivers may be more likely to commit looked but failed to see errors than novice drivers.

Although looked but failed to see errors are relatively common, there are techniques that can be used to minimise these errors occurring, and consequently the associated risks. Observation is a key skill for driving, but one that is often overlooked and taken for granted. Training can improve observation skills through developing visual search strategies.  Such methods can reduce the occurrence and risks that arise due to looked but failed to see errors.

[1] Langham (1999)

[2] Olson (1989)

[3] Hole et al. (1996)

[4] Underwood

Dr Victoria Bourne  (BA Hons, DPhil)

Consultant to Driving Risk Management Limited

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