Walking is a form of controlled falling with the body teetering on the brink of catastrophe as each leg swings through to save the ever-falling body mass. The slip is a result of a violation of a person’s expectations or assumptions by some aspect of the walking surface which may be unexpected. A fall occurs when there is insufficient friction between the weight-bearing legs and the walking surface. One of the most important factors influencing the incidence of slipping is the condition of the footwear/walking surface interface. The static friction between the pedestrian’s foot and the walking surface depends on the respective materials and on the area of contact between the two.
In order for the pedestrian to avoid a slip while walking, the horizontal and vertical forces applied by the individual must be resisted by forces acting equally against the foot as it contacts the walking surface. The most reliable component of this resisting force and the variable subject to manipulation is the coefficient of friction of the contact surface.
There is a greater danger of slipping and falling from an isolated small patch of ice. When a patch of ice is present on the walking surface it creates a lower coefficient of friction between the pedestrian’s footwear and the surface; and when the pedestrian’s forward motion applies a substantial horizontal force the foot will slip and the pedestrian will probably fall on such a surface. A pedestrian, who becomes aware of this hazard, may be able to limit the horizontal force contribution in order to balance the available frictional resistance of the ice and to walk safely. Adding sand or ice melting treatment to the surface will increase its coefficient of friction and allow for safe walking. Once the ice has melted, the higher coefficient of friction of the newly exposed surface will offer sufficient resisting force to permit walking without incident.
Research has shown that when a pedestrian can detect a very slippery condition they can compensate by adapting by shortening their stride length, reducing their foot’s velocity and thereby become able to maintain their body’s center of gravity and maintain stability. When the pedestrian is not made aware of a significant change in surface condition (ice with a lower coefficient of friction) and is unable to adapt to the new situation the pedestrian is likely to slip and fall. Ideally, with the perfect interface between the foot and the walking surface, one with no environmental contaminants, with the high coefficients of friction the pedestrian’s actions would not be a factor.
The entire slip and fall process takes significantly less than a second and without being given warning there is no time for the pedestrian to adjust to avoiding a slip (recover) and a potential fall.
The key stages that will lead to a slip and fall on an icy surface are:
1. Pedestrians approaches the slippery area
2. Slippery area is not visible to the pedestrian
3. Pedestrian steps onto the slippery area
4. Pedestrian experiences the slippery surface
5. Pedestrian recognizes the slippery surface
6. Pedestrian perceives a danger of losing balance
7. Pedestrian tries to avoid losing balance and slipping