4 SMIDSY – looked but not in clear vision

SUMMARY – when we talk about a driver’s field of view, we tend to assume that the eye acts like a camera and if a motorcycle falls within that area, the driver will see it… but the structure of the eye means that there’s no guarantee that a motorcycle within the field of view will actually be seen by a driver… clear, colour and focused vision only occurs across a tiny zone… the vast majority of incoming visual data falls into the fuzzy, colourless peripheral vision… but our brains create an the illusion of full-colour vision over a wide area… given the tiny ‘foveal zone’ the concept of  making ‘eye contact’ seems of doubtful value…


As we’ve seen on earlier pages, a great deal of work has taken place investigating just how ‘looked but failed to see’ crashes involving motorcycles happen. Although we generally think of conspicuity in terms of an object’s physical characteristics – that is, its size relative to other objects, its colour and its brightness against the background – this ‘sensory’ conspicuity is not the only factor in whether or not an object stands out. There is a second kind of conspicuity and it’s much more subtle because it depends on how the human brain processes the data the eye is sending. A motorcycle may be within the driver’s field of view (something commonly reported by witnesses to a car / motorcycle collision) but because the motorcycle COULD be seen is no guarantee it is ACTUALLY seen.

A key point of understanding is that the human eyes and brain are not the equivalent of the lens and the camera. The commonsense argument that “if it’s visible, we will see it if we look hard enough” simply isn’t true, as any stage illusionist knows. There are a number of reasons it is entirely possible to look towards, yet not perceive, objects within the visual field. It’s important to understand just why this happens. Here are three issues:

  • the eye’s foveal zone
  • saccadic masking
  • motion camouflage

Let’s start with the first. To give us the ability to detect detail within the visual field, the human eye is constructed in a way that allows us to focus full attention on just a tiny part of the background. Only a narrow cone in the centre of our field of vision is actually clear, focused, and full-colour vision because only a tiny patch of the retina – known as the fovea – actually transmits this camera-like image to the brain. To see detail, we need to orient our eyes so our ‘line-of-sight’ connects the fovea to the ‘fixation point’ (the focus of our gaze) in the outside world. Just how narrow our foveal vision is can easily be demonstrated. Make a ‘thumbs up’ sign. Look at your thumb nail… then look at your top knuckle. You’ll discover you cannot do this without physically shifting your gaze. That demonstrates the coverage of foveal vision is actually smaller than the size of our thumbnail at arms-length. In fact, it’s approximately two degrees of visual angle. So to get that clear, focused and full-colour image of nail and knuckle, we actually need to move our eyes to change the fixation point. Although few of us ever pay conscious attention to our vision, this phenomenon has been known to visual science for centuries – the discovery of the line-of-sight is attributed to Leonardo da Vinci.

Outside of the fovea, the image from the outside world falls on a part of the retina which has a very different construction – our peripheral vision. And here our view of the world changes – as Da Vinci observed, it turns increasingly blurry as well as black-and-white. Although with both eyes we have visual coverage which extends slightly more than 180 degrees left-to-right, the vast majority of the incoming visual data falls into our fuzzy, colourless peripheral vision.

Why does the human eye have this limitation? There’s a simple answer – transmitting the data to the brain. Processing ALL the visual data that falls on the retina to the same high fidelity as the fovea would require an optic nerve bigger than the eye – there simply isn’t the capacity to carry, let alone process the data. Interestingly, it seems designers of VR goggles have come across the same issue. To get a high pixel density – and thus high realism imagery – across the entire goggle would require more computing power than any domestic computer or phone can deliver. So they are trying to use this phenomenon to sharpen up the pixel density of the image ONLY where the user is looking. That way the screen provides increased resolution where the eye can see it rather than attempting to display it across the entire screen, frying the processor in the process.

Of course, we still have peripheral vision and in the experiment I just asked you to perform, you can still see all of your thumb (and your hand, and your arm and what’s behind it) but what you see lacks detail. Just 20 degrees off the line-of-sight, our clarity of vision (or ‘visual acuity’) is about one tenth of that of the fovea. What fools us is that our brains do an amazing job of stitching together visual input as we move our eyes, which gives the illusion of full-colour vision over a wide area. But it IS an illusion.

However, peripheral vision is good at is detecting light / dark contrast (particularly sudden bright stimuli) and movement. In either case, the involuntary response is to turn the head to bring the attractant into our line-of-sight so we can examine it with the fovea’s high-resolution vision. In terms of detecting vehicles on the road, that means we are likely to see big objects in our peripheral vision, but smaller objects – such as a motorcycle – are much harder to detect unless:

  • they have significant contrast against the background – this is what underpins conspicuity theory
  • they are moving laterally against the background – see ‘motion camouflage’

Crundall et all (2008) suggests:

“If one looks far off into the distance, then though the head turns and the eyes jump in the direction of the motorcycle, the subsequent fixation may still not land on the motorcycle if it is relatively close to the driver’s vehicle. As we shall see in the following section, the distance of any stimulus from the fixation point is crucial to detection. The fixation point describes the location in the world at which the most sensitive part of the retina is aimed. The acuity of the retina at this point is very high but covers a very small area. Objects which fall outside this area around the fixation point will be presented on a part of the retina with less acuity and therefore will be harder to detect.”

It’s also worth pointing out that the eye also has a lens, which focuses the image sharply onto the fovea. The real-life driving environment is 3D, something overlooked by many 2D studies using photograph and film. If a motorcycle is significantly nearer or closer than the object that first drew the driver’s attention, the bike will be out of focus. It takes time to refocus onto the new object of interest.

Foveal Zone

Incidentally, what about the eye’s retinal blind spot? Where the optic nerve enters the eye to connect with the retina (you can see it in the diagram) there is an area with no visual receptors, giving rise to the blind spot. It’s sometimes suggested that the blind spot might account for a driver’s failure to spot motorcycles. Whilst the other eye can usually see the area of where one eye is blind, we usually have a ‘dominant’ eye – close your eyes alternately and you’ll see what I mean. So it may be possible that an object in the blind spot of the dominant eye doesn’t ‘appear’ in the other eye either. However, the blind spot is also offset to the outside of our combined vision, away from the foveal pit which produces our cone of clear, colour vision. This means it falls within our peripheral vision. In theory, we could lose sight of a motorcycle within the blind area, but for that to happen it would need to be on a constant bearing AND we would need to be staring at something else and not moving our eyes, nor even our head. Whilst I would hesitate to say it never happens, for a bike to vanish in the blind spot right up to the point of collision would seem to require a rather unlikely set of coincidences.

One consequence of the tiny cone of colour, focused vision is that ‘eye contact’ proposed in motorcycle safety literature as a good way of ‘communicating’ between rider and driver seems a doubtful concept at best. The Canadian Thinking Driver website says:

“Eye Contact – The only way to know if another driver sees you is to make eye contact with them. If they are looking at you and you see them making eye contact with you, you can be fairly sure (but not guaranteed) that they see you.”

And a download for motorcyclists produced by Norfolk council says:

“In daylight try gaining eye contact with the driver.”

Yet anecdotally, I have heard motorcyclists say many times: “I had eye contact with the driver and he/she still pulled out”.  It’s happened to me. I believe that whilst we can make eye contact across short distances – say, across a room – it’s highly unlikely we can genuinely make eye contact approaching a junction, given the distances involved. Even in an urban context, the motorcyclist would need to make eye contact twenty or thirty metres away to be able to prevent the ‘looked but failed to see’ collision being set up. So whilst the driver’s eyes may be turned in our direction, there is a very real risk that his line-of-sight is fixated not on us, but on the more conspicuous car just behind us.

The best we can say is that if the driver is looking our way, we MIGHT have been seen, and if the driver is looking in some other direction then he/she almost certainly hasn’t spotted us. I searched for scholarly articles on ‘eye contact’ in driving and though I found numerous references in road safety literature, I found just one research paper concerning pedestrians making eye contract at crossing points. I believe motorcyclists would be wise to forget the concept of eye contact as a line of defence.

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References:

Anon, (2012) “Junction Problems” THINK Norfolk Partnership retrieved from http://www.suffolkroadsafe.net/assets/Motorcyclists/HuggerSuffolk/Hugger-PDFs/Hugger-Junctions-Guide.pdf

Crundall, D,. Clarke, D., Ward, P., and Bartle C. (2008) “Car Drivers’ Skills and Attitudes to Motorcycle Safety: A Review”. School of Psychology, University of Nottingham

Rothe, J. P., Cooper, P. J (1987) “Motorcyclists: Image and Reality” Transaction Publishers, 1987

Matthews, D. (2018) “Amount of pixels needed to make VR less crap may set your PC on fire” retrieved from https://www.theregister.co.uk/2018/01/16/human_limits_of_vr_and_ar/

McDonald, S., (?) “Watch out for motorcycles!” Thinking Driver – Fleet Safety, Vancouver Driver Safety Training retrieved from https://www.thinkingdriver.com/blog/tailgate-topics/19-watch-out-for-motorcycles

Last updated:

Wednesday 1 May 2019 – minor edit for clarity, typos fixed
Saturday 23 December 2018 – added comment about the retinal blind spot
Friday 23 November 2018 – minor edit for clarity, typos fixed

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