A comprehensive study of road safety (Treat et al., 1977) found that human error was the sole cause in 57% of all accidents and was a contributing factor in over 90%. In contrast, only 2.4% were due solely to mechanical fault and 4.7% were caused only by environmental factors. Other studies have reported similar results.
Why do humans make so many errors? The answer is much more complicated than just saying that the main cause was human error. Such studies are highly misleading. As Rumar (1982) notes:
"they tend At use human factors as a scrap box. Every accident behind which we do not find any technical error tends to be explained by the human factor."
This tendency is the result of a well known bias of human judgment to commit the "fundamental attribution error," or to vastly overrate human factors while vastly underrating situational factors when trying to explain why events have occurred.
Yet, it is often the opposite, as it is the situational circumstances that are primarily responsible for all accidents as they overwhelm the limited mental and physical aspects of the cyclist's information processing system. So, the key to understanding how this happens is to take a look an inside look on how a cyclist's information processing system works and how it could easily be overwhelmed in a hypothetical accident example.
So, to do this we must first take a look at what a cyclist must do. At anytime and under any condition during a ride, a cyclist must accurately and consistently...
- sense what is going on around the entire bicycle;
- make the right decision on what to At and when to do it; and, as a result,
- actuate the right control on the cycling vehicle in the right way to minimize or
prevent an accident from happening.
Along the way a cyclist riding down a busy street is bombarded with a steady flow of information. Most of the information is visual input, the road itself, other vehicles, pedestrians, signs, the passing scenery, etc. Moreover, the cyclist may be processing other information sources such as auditory input (listening to music on a headset, talking on a cell phone, carrying on a conversation with another cyclist), or internal input (remembering directions or planning what to make for dinner).
If the visual information flow is low, there may be enough mental resource to carry on all tasks simultaneously. But attentional demands may exceed supply when:
- the flow becomes a torrent (cycling faster)
- the information is low quality (poor visibility)
- resources must be focused on a particular subset of information (a car close ahead)
- the cyclist's capacity is lowered by age, drugs, alcohol or fatigue.
As a result, there may not be enough mental resource for all tasks. The cyclist then "attends" only a subset of the available information, which is used to make decisions and to respond. All other information, goes unnoticed or slips from memory.
In sum, the cyclist's information processing system works like this:
- The information from the visual and possibly auditory environment is detected by the senses (pre-attentive stage) while other information may be recalled from memory.
- If there is too much to process, the cyclist attends an information subset and the rest is ignored ("attentive stage").
- Lastly, the cyclist makes a decision and possibly a responses based on the attended information.
Due to these information processing system limitations research has shown that accidents occur for one of three main reasons.
- The first is perceptual error. Sometimes critical information was below the threshold for seeing - the light was too dim, the cyclist was blinded by glare, or the pedestrian's clothes had no contrast. In other cases, the cyclist made a perceptual misjudgment (a curve's radius or another vehicles speed or distance).
- The second, and far more common cause, is that the critical information was detectable but that the cyclist failed to attend/notice because his mental resources were focused elsewhere. Often times, a cyclist will claim that she/he did not "see" a plainly visible pedestrian or car. This is entirely possible because much of our information processing occurs outside of awareness. Mack and Rock (1998) have amazingly shown that we may be less likely to perceive an object if we are looking directly at it than if it falls outside the center of the visual field. This "inattentional blindness" phenomenon is doubtless the cause of many accidents.
- Lastly, the cyclist may correctly process the information but fail to choose the correct response ("I'm skidding, so I'll turn away from the skid") or make the correct decision yet fail to carry it out ("I meant to hit the brake, but I hit the gas").