For some of us, wintertime means braving cold temperatures and donning heavy winter clothes and boots. We seldom think of the hazards, but winter can be dangerous, leading to slips and falls on ice which may result in bad wrist fractures and even more serious hip and head injuries. The North West Public Health Observatory compiled data from UK hospitals that indicate that slips and falls on ice increase with colder-than-average wintertime temperatures.
Wearing appropriate footwear plays an integral part in wintertime safety. This means wearing shoes or boots with rough soles to provide much-needed traction. The Dunedin city council also advises residents to “put a pair of old socks over your shoes to increase grip” but this anecdotal advice remained scientifically untested.
Lianne Parkin, Sheila Williams, and Patricia Priest of the University of Otago, conducted randomized trials to test this hypothesis in 2008 and published their results. The group initially planned to recruit volunteers along Baldwin Street, the steepest street in the world according to the Guinness Book of World Records. The researchers deemed this location legally and ethically unwise and instead opted for two other sites used by university employees, students and members of the public on their way to work each morning.
The group received the 2010 Ig Nobel Prize in Physics for “demonstrating that, on icy footpaths in wintertime, people slip and fall less often if they wear socks on the outside of their shoes.”
Why is Ice Slippery?
We now believe that ice is slippery because of a quasi-liquid layer on the surface of ice. The water molecules at the surface in contact with air cannot properly bond with the molecules of the mass below, and are thus free to move like the molecules of liquid water. These molecules remain semi-liquid and provide lubrication, regardless of pressure exerted on the ice; demonstrating that friction is not necessary.
Testing Socks over Shoes to Prevent Winter-Time Falls
To test whether socks over shoes improve traction over icy footpaths, the group conducted a randomized control trial of 30 pedestrians traveling downhill on icy footpaths on the University of Otago campus. Fourteen participants wore acrylic-blend work socks while fifteen served as the control and did not wear socks. Participants were asked to report on how slippery they found their descents while assessors gauged and recorded how slippery the path seemed to be for each participant.
The study found that wearing socks significantly reduced the self-reported slipperiness of icy footpaths with a higher proportion of sock wearers displaying confidence as they descended the study slopes.