8 SMIDSY – looked but not perceived; workload

SUMMARY – when driving the brain needs to process sensory information… this is known as the ‘cognitive workload’… as driving tasks increase in difficulty, the workload starts to increase… even relatively simple tasks create an incoming information stream that exceeds the brain’s ability to process it all… stress in driving tasks further reduces the brain’s ability to process data… once the workload limit exceeded, a driver’s situational awareness at junctions is significantly degraded…

Back in the early 2000s, on my ‘blog before they were called blogs’, I wrote about ‘workload’ and military helicopters and how much the pilot actually had to do whilst flying the machine, navigating AND using the weaponry. In trials, it turned out that even hugely increased automation wasn’t enough for one design to be a single-seater as planned. There was simply too much going on even for a highly trained pilot. As a direct result, the Comanche helicopter was modified to carry two crew.

One of the key theories behind research into car – motorcycle collisions is termed ‘gap acceptance’, which seeks to understand how the driver calculates the ‘time to collision’ with an approaching motorcycle, decides whether or not a gap ahead of an approaching motorcycle represents a safe distance, and then makes a decision whether or not to pull out. However, the research usually pre-supposes a straightforward task – that there are no distractions and having seen the motorcycle, the driver simply estimates the motorcycle’s distance and speed to calculate ‘time to arrival’.

It wasn’t hard to predict that workload would also be an issue for the typical driver (or motorcyclist, come to that), who doesn’t have a fraction of the training of a helicopter pilot. In the article I predicted that in a complex driving environment, the driver would experience high workload. A common solution, adopted by many animals including primates (the group that includes humans), is to process just a small area or a few objects at any one time. We can then scan the visual scene in small chunks, subjecting each to more detailed visual analysis. This scanning technique has been compared to a ‘virtual spotlight’, highlighting different regions and objects for a closer look.

But it all relies on being able to slow down the scan. Pammer et al (2017) noted in their conclusions that:

“When we are driving, there is a huge amount of sensory information that our brain must deal with. We can’t attend to everything, because this would consume enormous cognitive resources and take too much time.”

In other words, the complexity of the driving task could lead to a disconnect between eyes and brain and workload offers at least a partial explanation for ‘looked but failed to see’ collisions between a motorcycle and a car. Some of the visual information within the scene would simply not reach the conscious, thinking part of the driver’s brain. Focused on one visually-intensive task – perhaps searching for road markings indicating the correct lane on a busy, complex roundabout – other visual input goes missing and the driver loses track of the motorcyclist. Even when the driver looks in the direction of the oncoming motorcycle – in some cases appearing to look directly at the motorcycle – the motorcycle goes missing and the driver pulls out into its path.


Picture yourself emerging from a side turning then turning right onto a busy road – the situation in which the classic SMIDSY collision occurs. We’re searching for, then monitoring, multiple moving objects, which are travelling at different speeds in at least two different directions – three if the junction is a cross roads. We’re looking in several different locations – the two lanes of the road itself, the nearside margin of each lane (where bicycles might be expected), plus pavements to check for pedestrians. We need to detect stationary objects which are in our lines-of-sight and be aware how they might create blind spots. We need to move our eyes from one search zone to another to visually acquire the targets to be scanned. Each eye movement (a ‘saccade’) takes time, and then the eye has to refocus on the new scene, which also takes time. And we should be keeping an eye on the mirrors too.  ‘Looking properly’ is far from a trivial task.

Stress is known to affect our ability to process information. Trying to follow complex road layouts in an unfamiliar town, we rapidly move towards a condition of stress where we are not able to process as much information. Once the workload exceeds the level we can handle, the results include what are termed “compensating behaviours”:

  • errors – we make the wrong decision
  • slow task performance – it takes a long time to reach the right decision
  • task shedding – we never make a decision
  • rapid task switching – we keep mentally jumping from one part of the overall task to another

Gap acceptance.png

In the worst case, we might fail to perform a task altogether, a condition known as ‘task shedding’. From working in motorcycle training, I know how novice riders who are experienced drivers can actually forget a ‘simple’ task like looking for conflicting traffic when pulling out of a junction.

Few of us are consciously aware of just how much workload even relatively simple tasks create. A study by Murphy and Greene in 2016 put forty two drivers into a life-size Volkswagen Polo driving simulator where they performed a series of gap perception tasks involving judging if their vehicle could fit between two parked vehicles. There were cars parked on either side of the simulated road. When the gap between the parked cars was easy to negotiate, 22 of 41 drivers noticed an unexpected pedestrian in a red blouse. But when drivers the gap was reduced so that it was barely wide enough for the car to negotiate, only seven noticed the pedestrian.


Passing between two parked vehicles is a trivial task compared with monitoring busy roads to decide if it is safe to turn. It should be clear that excess workload can significantly affect a driver’s situational awareness at junctions and in some instances a failure to spot other vehicles (including motorcycles) isn’t ‘lack of attention’, it’s ‘not enough to go round’. However, hardly any quantitative work has been done to investigate this. The authors point out:

“This study is the first to demonstrate perceptual load effects on awareness in an applied setting and has important implications for road safety”.

So next time someone says that “drivers should look harder for bikes”, just have a think about how complex the task actually is. The surprise is not that drivers fail to spot motorcycles (and other vehicles) but the fact that they spot them many, many more times than they don’t.

Since you’re here, I’ve a small favour to ask. If you feel able to make a small donation to the upkeep and continued development of SOBS, why not buy me a coffee? Each contribution is much appreciated. Each cuppa keeps me awake and writing! Thank you.


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Helman, S., Weare, A., Palmer, M., Fernandez-Medina, K. (2012). “Literature review of interventions to improve the conspicuity of motorcyclists and help avoid ‘looked but failed to see’ accidents”, Published Project Report PPR638, Transport Research Laboratory Road Safety Group.

Murphy, G., Greene, C., M. 2016 “Perceptual Load Induces Inattentional Blindness in Drivers”

Pammer, K., Sabadas, S., Lentern, S. (2017) “Allocating Attention to Detect Motorcycles: The Role of Inattentional Blindness. Human Factors”: The Journal of the Human Factors and Ergonomics Society


Last updated:

Friday 23 November 2018 – minor edit for clarity



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