Slips, Trips and Falls: How Well do We Really See Tripping Hazards?

The short answer is:  not very well.

I am an avid runner.  In my legal practice I’ve had the opportunity to work with a lot of runners who have had bad trip and fall injuries while running.  As a runner, I’ve always been fascinated by what causes these falls.  When I take on one of these cases, I have to understand what caused the fall and why didn’t my client see the hazard he or she tripped over.

This weekend, while out on a trail run, I got to thinking about this while I was navigating a particularly root infested Pacific Northwest trail.  I noticed that despite the fact that I was running through a beautiful forest, I was not actually seeing much of it.  My eyes were entirely focused on the ground immediately ahead of me.  I compared this experience to a normal run on the road.  There, I noticed my eyes were typically focused farther ahead and I took in much more of my surroundings, but with less attention to the ground right in front of me.

It turns out the limitations I noticed in my perception are normal and my experience highlights why many of the runners I’ve worked with over the years have suffered a trip and fall injury on something “that was there to be seen.”  We generally “see” and perceive much less than we think we do.  This phenomena that has been well studied explains why people frequently trip over things there are “available” to be seen – but they miss them anyway.

Here is a diagram of our normal field of vision:

You can see that normal line of sight is around 15 degrees.  Our eyes will rotate to about 30 degrees.  Beyond 30 degrees we have to rely on peripheral vision (which has its limits) or actually moving our head to point the eyes at what we want to see.  We have similar limitations in both horizontal and vertical planes.  The net result of our eyes natural field of vision is that we actually have only a relatively small area in the center of our field of vision where we see at high resolution.

Its true that our peripheral vision has a fairly wide range.  However, humans have a relatively weak peripheral vision overall.  Our peripheral vision is poor at distinguishing colors, shapes and detail.  It does ok at detecting motion.  Generally, its little help in seeing and perceiving trip hazards.

Visual field is also effected by speed.  Our field of vision actually narrows as speed increases.  This means that the area we see and perceive with detail is even smaller.

Finally, studies have demonstrated that there’s a difference between seeing something and perceiving something.  Visual perception is the ability to interpret and understand your environment.  In the context of a runner avoiding a trip hazard, this means that the runner will have to detect the trip hazard within the field of vision, interpret it, understand it requires a reaction, and finally, actually react to avoid it.  This loop has been shown to take anywhere from about 1 – 1.5 seconds to occur.

Our limited field of vision explains why thresholds and elevation changes are often painted or marked with a bright color.  The color is picked up by the peripheral vision as something out of the ordinary.  This draws the eyes to the feature so that it is actually seen and perceived.

Thus, for a runner, the combination of increased speed and a relatively narrow field of vision can mean that there are a lot of things we are not seeing that we could potentially trip over.  It also explains why I wasn’t taking in any of the forest around my on my run this weekend.

So, if you are running and accidently trip over something that in hindsight looked pretty obvious, don’t feel so bad about it.  You are probably just experiencing one of the limitations of our natural field of vision and perception ability.

Run safe and keep your eyes open!