SUMMARY – many early studies implicated poor saliency and poor sensory conspicuity as the reason for ‘looked but failed to see’ errors… the narrow frontal shape of a motorcycle makes it harder to spot than a car… many studies have focused on finding ways of making motorcyclists ‘more conspicuous’… but despite collision statistics remain largely unchanged… this suggests that it may not be a lack of conspicuity that is the likely explanation for car drivers missing motorcycles… but poor motorcycle conspicuity has become accepted as fact rather than asking whether the hypothesis is actually correct. 

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So far I have dealt with two explanations for the ROWV that results in a collision between a motorcycle and another vehicle; ‘did not look’ and ‘looked but could not see’. In the first case, the driver simply failed to check in the right direction and in the second the motorcycle was physically out of sight from the driver’s perspective. But not looking or being unable to see the motorcycle doesn’t explain all collisions. Even when there is a line-of-sight between the driver and the motorcycle, and the driver looked in the right direction, things still go wrong.

Crundall et al (2008) characterised the two main error types as:

• detection errors – ie, a failure to look at and identify the presence of a motorcycle
• decision errors – ie, a failure to correctly appraise the speed and /or path of an approaching motorcycle

I’ll be looking at the ‘looked saw and misjudged’ decision error in the next section, so for now let’s try to understand how the ‘looked but FAILED to see’ detection error can happen.

It’s long been known we are more likely to notice – and thus remember and make use of – some kinds of information more than others. Although it’s not the only way that an object can be conspicuous (we can smell or hear too), we generally think of conspicuity in terms of an object’s visual characteristics – that is, its size relative to other objects, its shape, and its colour, luminance and its brightness against the background compared to surrounding objects.

Objects that attract our attention and easily detected are known as having ‘high saliency’ and is a measure of an object’s ability to attract the observer’s attention. The important image characteristics are luminance contrast, colour, and edge orientation. A good example of objects that have high visual salience – thus attracting our attention – would be colourful round red berries on a background of oval green leaves. This visual distinction of one object within the environment has been termed ‘sensory’ conspicuity and it’s been theorised that the human brain has evolved to detect salience.

As vision is our primary sense when driving, how easily seen or noticed a particular object is an important issue for all road users. We may pick out highly salient objects as a result of scanning a scene, or our attention can be drawn involuntarily towards salient objects – for example, by the sudden onset of a bright stimulus. When our visual system detects something with visual salience, a signal goes to the real-time brain. Whatever caught our attention pops up to the level our consciousness and we become actively aware of whatever the visual system has detected.

In the earliest studies of the ‘looked but failed to see’ collision between a car and a motorcycle, it became clear that even when witnesses reported that a motorcycle was in plain sight and accident reconstructions concluded it was visible from the driving seat of the car, the drivers often said they didn’t see the motorcycle. It’s not therefore entirely surprising that many early studies implicated poor saliency and poor sensory conspicuity. Many of the earliest expressions of this ‘conspicuity theory’ came from Hurt. For example, in 1977, Hurt and Dupont considered that:

“When the motorcycle and the automobile are on collision paths, or when the vehicles are in opposing traffic, the conspicuity due to motion is very low, if it exists at all. Consequently, recognition of the motorcycle by the automobile driver will depend entirely upon the conspicuity due to contrast.”

This supposition was repeated in a later study by Hurt et al (1984):

“The majority of motorcycle-automobile collisions are the result of a violation of the motorcyclist’s right-of-way by the automobile driver. The predominating cause of the multiple vehicle collision is a failure of the automobile driver to detect the presence of the motorcyclist in traffic when a traffic conflict develops. Such detection failures are associated with low conspicuity for the motorcyclist…”

It’s widely accepted that since motorcycles have poor sensory conspicuity and where the environment contains other, more conspicuous objects such as nearby cars, the driver’s attention is likely to be drawn towards that object rather than the less conspicuous motorcycle, which is thus more likely to be overlooked, resulting in a delayed response by the car driver or even a complete detection failure. Studies looking at gap acceptance – whether or not another driver will turn in front of another vehicle – which have shown that drivers are more likely to accept turn into smaller gaps ahead of motorcycles than other vehicles have been used to support the low visual salience theory (although there are other explanations, as we’ll see). An approaching car is also more likely than a motorcycle to generate an ‘eye fixation’ immediately before the driver makes a decision to turn.

So what makes motorcycles tough to spot? Crundall et all (2008) suggested that:

“One particular bottom-up influence seems especially relevant: spatial frequency (the width of the vehicle). Global Precedence theory suggests that we extract low spatial frequency items from a visual scene first (including wide vehicles such as cars). Thus we are more likely to miss narrow motorcycles, which are considered to be high spatial frequency items.”

You’re probably wondering what spacial frequency means. I had to look it up too, but in essence, it refers to the level of detail present across the horizontal visual angle of the object – its width, if you like. The motorcycle’s vertical edges are close together. By contrast, the car’s vertical edges are much further apart. Essentially, the very shape of the PTW makes it harder to spot than a car.

Given the saliency issue, it’s not surprising that finding ways of making the motorcyclists ‘more conspicuous’ has been the subject of numerous investigations. By and large, the results appear to demonstrate that light-coloured clothing and headlights do make motorcycles stand out better. These observations have been used to advocate the use of day riding lights and high-visibility clothing for motorcyclists, on the basis that drivers are more likely to be cautious when a visually-salient motorcycle is present, reducing collisions, thus resulting in fewer injuries. You’ll remember I mentioned the mid-70s Greater London ‘Right Bright’ campaign that was probably the first to encourage motorcyclists to improve their conspicuity by wearing bright clothing, preferably of fluorescent material, and by switching on their headlights in daytime.

But what about the test conditions? It should be obvious that whether or not a motorcyclist stands out depends at least partly on what is behind the rider. What matters is not so much whether the rider is using light coloured clothing or a low beam headlight so much as whether that creates contrast between rider and the background. Many of the studies – which are still quoted and still used as evidence for the effectiveness of conspicuity aids – placed the rider against a relatively uniform backdrop.

There was a second problem – motorcycles are usually moving in front of other moving vehicles, and many of these studies – particularly the early ones – used photos which required the subjects to detect a static motorcycle against a static background. More recent studies have become aware of these limitations. For example, Pai in a literature survey published in 2011 noted that:

“A substantial number of studies have manipulated physical characteristics of motorcycles and motorcyclists to enhance conspicuity… Although various conspicuity aids have proven effective, some researchers reported that motorcyclist’s/motorcycle’s brightness per se may be less important as a determinant of conspicuity than brightness contrast between the motorcyclists and the surroundings.”

In their book on motorcycle conspicuity, Roessger et al (2015) state:

“PTW conspicuity may be related to the motorcycle, to other vehicles, to the riders themselves, to other drivers, to the road environment or any combination of these factors… for example, riders can use daytime riding lights to decrease their probability of collision with another vehicle, but they do not control ambient traffic conditions… most importantly, conspicuity is not constant but changes with the time of day, the weather conditions, the urban environment, the presence of absence of other road users.”

In other words, the most conspicuous clothing and lighting changes moment-by-moment depending on the and local environment – there is no straightforward ‘one size fits all’ solution.

The crucial question is whether or not conspicuity aids have actually reduced collisions at junctions. Almost thirty years after Hurt, Underwood et al (2011) continued to argue that:

“Road safety observations have concluded that highly visible road users are less likely to be involved in crashes, suggesting that saliency is important in real-world tasks.”

On the other hand, actual crash statistics don’t support that conclusion. Given the widespread use of conspicuity aids, there’s no obvious reduction in the proportion of collisions occurring at junctions, which is what we could expect were conspicuity aids effective. Sager et al (2014) noted:

“…collision statistics remain largely unchanged, suggesting that the issue may not be related solely to the motorcycle’s static properties.”

The first study I have found that questioned the conspicuity theory was written as long ago as 1989. Olson challenged motorcycle conspicuity as the likely explanation for car drivers missing motorcycles and found “it lacks empirical support”. He observed that:

“The conspicuity hypothesis has not been seriously challenged. Almost all investigators have accepted it as fact, concentrating their efforts on means to improve conspicuity rather than on asking whether the hypothesis is correct. This is unfortunate because alternative hypotheses can be advanced. Some have research data to support them; some are speculative. All are consistent with the known facts…”

I believe that we should conclude that road safety advice that relies entirely on the conspicuity theory must be treated with some caution, and categorical statements such as “motorcyclists should wear hi-vis kit and use day riding lights to be more visible” should be seen as inaccurate since there is clearly no ‘one size fits all’ solution.

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References

Chih-Wei Pai Motorcycle right-of-way accidents – A literature review Accident Analysis & Prevention Volume 43, Issue 3, May 2011, Pages 971-982

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

Hurt, H.H., DuPont, C.J. (1977), “Human Factors in Motorcycle Accidents”, SAE Technical Paper 770103, doi:10.4271/770103.

Hurt, Jr., H.H., Hancock, P.A., Thorn, D.R. (1984) “Motorcycle-Automobile Collision Prevention through Increased Motorcyclist Frontal Conspicuity” Proceedings of the Human Factors and Ergonomics Society Annual Meeting

Roessger, L., Lenne, M., G., Underwood, G. (2015) “Increasing motorcycle conspicuity – design and assessment of interventions to enhance rider safety” Routledge Press

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

Underwood, G., Humphrey, K., van Loon, E. (2011) “Decisions about objects in real-world scenes are influenced by visual saliency before and during their inspection”; Vision Research Volume 51, Issue 18, 15 September 2011, Pages 2031-2038

Last updated:

Wednesday 1 May 2019 – minor edit for clarity and to fix typos

Friday 23 November 2018 – minor edit for clarity and to fix typos

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