SUMMARY – the human visual system is sensitive to lateral movement across the background… but we are particularly poor at spotting things moving directly towards us… this is known as ‘motion camouflage’… motorcycles approaching a driver waiting to turn may not create any lateral movement… and the driver fails to spot the bike due to motion camouflage…

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We’ve seen how visual conspicuity is not the only factor to consider when looking at collisions between cars and motorcycles. We’ve also seen that the vision system has two particular limitations – the eye’s foveal zone and the phenomenon known as saccadic masking. So now we’ll turn to a third perceptual issue – motion camouflage. Motion camouflage is actually a topic that I was previously aware of, because it comes from my own field of study, the biological sciences. It’s well-known that hunting animals stalking prey will approach along a line that keeps them motionless relative to the background from the perspective of the prey animal. If the prey animal moves, the hunters subtly adjust their own paths so that they stay in that same relative position. This is how big cats operate. Some male insects use the same technique to sneak up on a potential mate. The hunter can get surprisingly near before it is close enough to be detected by ‘looming’ – but this only happens when it is now so close that it fills the background.

It is lateral (ie, side-to-side) movement that usually helps us detect an object at a distance because it’s difficult to detect movement via change of apparent size. Artists exploit perspective – if they decrease the apparent size of an object, the distance to that object appears to increase. It works because we hold in our head a kind of ‘reference size’ which allows us to quickly estimate distance. There’s a simple rule artists follow – to make an object appear twice as far away, it must be one-quarter the original size. Of course, this works the other way. A small, distant object that starts small stays relatively small even as the distance is closing down.

So this stalking technique works because the hunter avoids lateral movement. The only clue is that the apparent size of the object increases as it gets closer, but just as the prey fails to spot a big cat until the last moment, drivers often fail to detect other vehicles until they are right on top of them. Even buses, as bus drivers will tell you. A moment’s thought should tell us that a motorcycle approaching a vehicle that is waiting to turn at a junction is on also on an approach path that barely moves relative to the background, and so also suffers motion camouflage. It’s only when the bike is almost on top of the driver that it is so close that it fills the background – the phenomenon known as looming. This size / distance issue is well-known to artists, but is ignored in safety literature.

How important is motion camouflage? Sager et al (2014) stated:

“A driver turning left* and failing to notice an oncoming motorcyclist until too late is the most common cause of motorcycle collisions. Consequently, much previous research has focused on motorcycle properties, such as size, shape, and color to explain its inconspicuousness. However, collision statistics remain largely unchanged, suggesting that the issue may not be related solely to the motorcycle’s static properties.”

(* whilst driving on the right)

To test this, the team used a simulator to investigate the effect of lane position on whether or not an oncoming driver would turn in front of a motorcycle. They found that:

“…drivers are more likely to turn in front of an oncoming motorcycle when it travels in the left-of-lane position than when it travels in the right-of-lane position”.

It’s important to understand that this study looked at drivers turning INTO a side road across the motorcycle’s path, not the better-known SMIDSY collision with an emerging driver. The left-of-lane position therefore places the motorcycle near the centre line of the road. By shifting to a right-of-lane position, the motorcycle is positioned further away from the potentially turning vehicle. It’s logical to deduce that opening the angle in this way would increase the apparent rate of movement of the motorcycle against the background as it closed on the driver’s position. I’ll come back to this idea later.

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References:

Sager, B., Yanko, M., R., Spalek, T., M., Dastur, F., N. (2014) “Motorcyclist’s lane position as a factor in right-of-way violation collisions: A simulator study”. Accident Analysis & Prevention 72C:325-329 · August 2014

Last updated:

Wednesday 1 May 2019 – minor edit for clarity, added section on perspective

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