By George Melchoir, ASM
In 2014, unintentional fall-related injuries were the leading cause of unintended emergency room visits in the United States, accounting for over 9 million injuries, according to the U.S. Centers for Disease Control and Prevention (CDC). In the same year, the U.S. Bureau of Labor Statistics (BLS) reported that over 40,000 workers lost time at work due to ice-related slip-and-fall injuries. Despite education and emphasis on fall protection across the industry, a review of the annual data reveals that numbers in the CDC and BLS injury categories continue to increase annually. More concerning, the snow and ice management industry at large continues to exercise an understated, if not absent, approach to affirmative ice management on walking surfaces. To reverse the trend of ice-related injuries, and the costs associated with those injuries, owners and operators must consider human factors associated with pedestrian movements across their properties when developing their snow and ice management plans.
The human factors
Human factors are our physical and behavioral tendencies that influence how we interact within a system, such as the built environment. Human factors are considered in design and operations of various products and systems such as smartphones, automobiles, roadways and buildings. The two human factors most prominent in wintertime operations of pedestrian facilities are vision and the human gait.
The optical environment in which they see objects and discern between them in their visual settings influences how and where a person walks. The optical environment is experienced through vision. Basically, there are two parts to human vision: foveal (central) vision, and peripheral vision. Foveal vision is a narrow vision that comes from the center of the retina and is the part of our vision that we use to maximize detection of detailed information in the optical environment, including object characteristics such as shape, texture, aspect and color. Peripheral vision has a much wider aperture and a lower resolution. Our peripheral vision is insensitive to color and detail. Instead, we use our peripheral vision to detect objects of high contrast, or objects in motion, which we then shift into our foveal vision to ascertain detailed information.
If a person is walking across a parking lot to their car at quitting time, assuming that the lot is clear and has been treated, they are more apt to walk with their heads up so as to train their foveal vision toward oncoming vehicles or their destination. In doing so, the person leaves the detection of unforeseen ice and other hazards to peripheral vision, which is not suited to detect the subtle indications of the presence of ice. Icy conditions on walking surfaces are difficult to discover with peripheral vision because subtle anomalies associated with ice, such as light reflection on the icy surface, or changes in color caused by entrained air in the ice, do not provide enough contrast for peripheral detection. Detection of ice is even more difficult on overcast days, or during non-daylight hours, both of which are plentiful in the winter.
Mechanics of gait
The other contributing human factor to ice-related pedestrian injuries is the human gait, which is the measured pattern of movement of one’s limbs as they move across a surface (e.g., walking), and is largely a function of their base of support. Standing upright, a person’s base of support is the area under and between their feet and supports the person’s weight. The point through which the entire weight of a body is concentrated is their center of gravity, which is generally just inside and above one’s navel and is situated in the middle of the base of support. Contrary to standing, when walking across a level surface, we shift our center of gravity in front of our base of support to initiate and maintain motion in the forward direction. We apply a pushing force at the point where the foot meets the walking surface. With the center of gravity forward, and our planted foot pushing to the rear, the body will temporarily be in an unstable position. Therefore, to maintain an upright posture when walking, a person’s stability becomes a function of their ability to reestablish their base of support with each step. This motion is generally described by four successive phases of a gait: heel strike, midstance, toe off, and swing.
The gait cycle: After toe off, leg swings through for the next heel strike. In the presence of ice, the lake of friction causes the heel strike to fail by allowing the heel to slide instead of plant.
When stepping on an unforeseen, undetected icy surface, the most susceptible phase of the gait for loss of balance is the heel strike. As a person walks across a surface, such as a parking lot, he is shifting his center of gravity forward in his base of support with each phase of his gait, and so his weight shifts from foot to foot. The point that weight is shared and shifted is the re-establishment of the base of support, which is when the heel contacts and acts as a stable pivot point to shift body weight onto the forward foot. If the heel is to act as a weight pivot point, it must be firmly situated on the surface. Because the heel of a shoe or boot is moving forward and away from the body on contact with the surface, a person must rely heavily on friction between the heel of their footwear and the walking surface to create firm, stable contact. It is the lack of friction inherent of icy surfaces that causes the heel strike phase of the gait to fail by allowing the heel to slide instead of plant. When that happens, the body is placed in a prolonged state of instability as the center of gravity is shifting, which results in one’s feet slipping out from under them such that they fall sideways or backwards before impacting the surface.
Mechanics of human gait: The body’s center of gravity is generally just inside and above one’s navel and is situated in the middle of the base of support. When walking across a level surface, we shift our center of gravity in front of our base of support to initiate and maintain motion in the forward direction.
An average person who slips and falls backwards on undetected ice will strike the ground with approximately 2,000 pounds of force, more than enough impact force to tear ligaments, separate joints and fracture bones.
As such, property owners and operators should consider pedestrian vision and gait in their ice management plans for walking surfaces:
- Surfaces should be well lit and should not rely on distant lighting that will have low angles of reflection off of icy surfaces.
- Lighting should be uniform and absent of glare at the pedestrian level.
- Ice mitigation and treatment plans should have a deliberate focus on maintaining adequate friction between footwear and the walking surface through the prevention of ice formation.
- Inspection and maintenance of walking surfaces should be an ongoing endeavor rather than just a component of post-storm cleanup and treatment.
- Create designated walkways or changes in the mode of operations so as to reduce distraction during pedestrian movements.
George Melchior, ASM, is a registered architect and professional engineer and owns GVM Consulting, based in Portsmouth, NH. Contact him at firstname.lastname@example.org.