FORENSIC HUMAN FACTORS ANALYSIS OF SLIP AND
FALL INCIDENTS USING A VARIABLE INCIDENCE TRIBOMETER
Terence J Fischer and Jamie S A Fischer
Fischer Forensic Services, LLC
Northfield, NJ 08225 USA
This paper addresses how the application of the Human Factors Forensic Accident
Analysis Model and appropriate tools, such as the Variable Incidence Tribometer,
provide evidence for expert opinions regarding the causes of slip and fall incidents.
This paper addresses the biomechanics and causes of slip incidents and thetechniques used in measurement of slip resistance in the laboratory and in the field.
This approach has been used successfully in forensic analyses and has been
accepted for expert testimony by United States Federal Courts and State Superior
Courts. Slip falls occur when there is a loss of balance by the pedestrian as a resultof too little friction between the foot and the floor. The slippery conditions are
frequently caused by the introduction of contaminants between the shoe and thefloor surface. Water and other liquids are frequent contaminants that lead to slip
and fall accidents. Applying an Accident Analysis Model is useful in thedetermination of slip and fall accident causes. The Accident Analysis Model used
has five major elements: Design, Remove, Guard, Warn, and Train. Field
measurement of the COF can be accomplished using several devices. The Variable
Incident Tribometer (VIT) is designed to measure static COF of dry surfaces(Johnson, 2004.) The presence of a lubricant, i.e., water, between the shoe and theground surface contradicts the definition of COF. In that condition, the slip
resistance can be measured by the VIT. The VIT provides readings that areconsistent with the judgments of people walking on various surfaces with differinglevels of slip resistance. Data from the VIT has been accepted as evidence by stateand federal courts in the United States of America.

Introduction
Slips and falls in the workplace and commercial establishments are a primary concern for safety
experts. According to the National Safety Council (NSC), in 2007 more than 7.9 million Americans
were injured and more than 21,700 Americans died as a result of falls (NSC, 2010).
Slips and falls occur when there is a loss of balance by a pedestrian as a result of too little
friction between the foot and the floor. The slippery conditions are typically caused by the
introduction of contaminants between the shoe and the floor surface. Frequent examples ofcontaminants include water, other spilled liquids, and the certain floor finishes.
Two types of slips can occur during normal walking. The heel of the swing foot can slip
forward when placed on the ground or the planted foot can slide rearward. The probability of either
type of slip is increased in the presence of contaminants.

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Accident Analysis Model
The Accident Analysis Model can be used to determine the cause and prevention of slip and fall
incidents. The Accident Analysis Model has five major elements: Design, Remove, Guard, Warn,
and Train (Askren & Howard, 2004).
Design. The design of the environment contributes greatly to the likelihood of slips andfalls. Therefore, when a slip and fall incident is evaluated, the first step must always be tothoroughly evaluate the design of the environment. It is important to determine the source of thecontaminant and whether that contaminant would be expected in the subject environment. A goodevaluation will address any potential design changes that would eliminate or minimize the presenceof contaminants. For example, entranceways from the outside are frequently exposed to rainwater
and, thus, produce a potentially hazardous condition. The use of canopies would minimize
exposure to the contaminant and reduce the likelihood of slips and falls. If the designer had
included a canopy, it is likely that the incident would not have occurred.
Remove. The removal of the contaminant from the environment would greatly reduce theprobability of a slip and fall incident. If a design modification is not feasible, the next best stepwould be to evaluate whether the contaminant could have been removed prior to the incident. For
example, if there was an accidental spill, could the spill have been mopped dry prior to the incident?
How long was the spill allowed to remain on the floor? This element of the Accident Analysis
Model typically involves the evaluation of a property manager’s policies and procedures. It isuseful to obtain policy manuals and interview employees.
Guard. Guards can prevent the pedestrian from exposure to the contaminant. According to
the Accident Analysis Model, there are situations where contaminants can be expected in theenvironment and it is not feasible to remove the contaminant in any timely manner. In many ofthese situations, it is feasible to protect the pedestrian when these hazards are present. For example,
the use of bars and railings could provide a handhold that will guard the pedestrian against potential
falls.
Warn. If the hazard cannot be prevented and if the pedestrian cannot be guarded, a prudent
property manager will warn pedestrians of the potential hazard. A forensic evaluation will
determine if, for example, warning cones and signs were used to inform pedestrians of temporary or
unpreventable hazardous conditions. The warning will enable the pedestrian to make aknowledgeable decision about whether to proceed on the walkway.
Train. The final element of the Accident Analysis Model is to train a person to safely
operate in a known hazardous environment. This element is often applied to many workplacesituations. Training however, is not an appropriate countermeasure for slip and fall incidents
involving the general public.

Slip Resistance Measurement
Slips and falls occur when there is a loss of balance by a pedestrian as a result of too little friction
between the foot and the floor. The actual COF (Coefficient Of Friction) is quantified on acontinuum from 0 to 1.0, on a flat surface. Studies have shown that the required COF for a person
to walk without slipping is typically around 0.30 to 0.40 depending upon gait and other individual
factors (Brungraber & Templer, 1991). The Underwriter’s Laboratory established in the early
1940’s that a COF of more than 0.5 was considered slip resistant (Sacher, 1993). This value has
been repeatedly cited and is accepted in the flooring industry as the standard performance measure.
Field measurement of the COF can be accomplished using several devices. The Variable
Incident Tribometer (VIT) is designed to measure static COF on dry surfaces (Johnson, 2004). The
presence of a lubricant, i.e. water, between the shoe and the ground surface contradicts the- 236 -

definition of COF. In that condition, the slip resistance can be measured by the VIT. The VITprovides readings that are consistent with the judgments of people walking on various surfaces withdiffering levels of slip resistance. The slip resistance measurements of the VIT have been accepted
as evidence by United States Federal Courts and State Superior Courts.
The VIT produces consistent slip resistance readings that are independent of the flooringslope. Adjustments can be made mathematically to the slip resistance measurements toaccommodate ramped surfaces. Unlike pull meters, the VIT has no appreciable residence time on
the lubricated surfaces. This prevents the occurrence of adhesion or stiction, which can provide
spuriously high slip resistance readings.

Examples of Cases.
The following section will present cases where the VIT was successfully used by the author in aforensic capacity.

Case 1: Poor Restroom Design Causes Hazardous Flooring Condition

In the first case, the plaintiff was a lunchtime patron that had used a restroom of a restaurant. Afterleaving the toilet stall, the plaintiff approached the sink counter to wash her hands. The plaintiff did
not notice that the porcelain tile flooring was wet in the area near the sinks. The plaintiff waselderly and used a cane to assist her in walking. The plaintiff’s cane slipped and she fell to theground injuring her head, wrists, and knees.
Measurements of the slip resistance of the flooring were taken using the English XL VIT.
The testing results indicated that when dry, the tiles had a slip resistance of 0.58, therefore slip
resistant. When wetted with local tap water, the slip resistance measured 0.31 and was considered
not slip resistant and by code it was classified as hazardous.
Testimony indicated that the plaintiff was wearing flat soled shoes and was walking in adeliberate fashion. Examination of the cane indicated that the tip had a good quality rubber tip.
Witnesses testified that two other patrons were in the restroom at the time of the incident
and had just finished washing their hands. An inspection of the restroom indicated that two sinks
were located in the countertop. The restaurant installed a paper towel dispenser on the left side wall
approximately 2 feet (0.61 meters) from the left sink. Patrons using the right sink had to walkapproximately 6 feet (1.83 meters) to obtain a towel after washing their hands. This resulted inright sink users dripping water on the floor as they walked to retrieve a towel.
The restaurant management testified that the restrooms were cleaned every morning before
opening. There was no scheduled maintenance or inspection of the restaurant during the day/lunch
shift. The management testified that the lunchtime clientele was an older crowd with 75% to 80%
over the age of 62. Many of the patrons used canes or walkers. The restaurant typically served a
few hundred patrons during the lunch shift. After 4:00 p.m., an attendant is tasked with maintaining
the restrooms. The attendant wipes the floors and replenishes the supplies at that time.
It was the opinion of this investigator that the restaurant did not provide an adequate
standard of care in designing, maintaining or warning the patrons of the potential hazard caused by
the slippery wet floor condition. The application of the Accident Analysis Model indicates thatapplication of the first (Design), second (Remove) and fourth (Warn) principles highlights the
deficiencies. These principles would have been feasible to implement yet they were not.
The design of the restroom led to the hazardous conditions. The placement if the towel
dispenser on the wall opposite the sink caused patrons to drip water onto the floor. Review of ADA
standards and reference safety texts identified the unsafe condition caused by the improper designof the subject restroom, specifically the installation of the towel dispenser. The use of flooring thatwas not slip resistant when wet exacerbated the condition. The behavior by the patrons was not
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the restaurant had no policy for maintaining or inspecting the restroom during the lunchtime hours
when hundreds of patrons were potentially exposed to the hazardous condition. Had routine
inspections been performed and necessary maintenance activities been performed, the wet flooring
condition could have been removed. Finally, had the restaurant warned patrons of hazardous
conditions, through the placement of signs, the plaintiff could have adjusted her path and/or her gaitto prevent her exposure to the hazardous condition.

Figure 1. Case 1 incident location. Note the distance from the right sink and thetowel dispenser at left.

Case 2: Safety “Diamond Plate”

The plaintiff was a vacationer at a seaside resort town when a slip and fall accident occurred. The
plaintiff was walking on the seaside boardwalk when the weather changed and rain began falling.
The boardwalk was crowded with patrons all hurrying to escape the rain. The wet boardwalk
surface was an amalgamation of concrete and wood with a “diamond plate” aluminum manhole
cover located in the man walkway path. The plaintiff unknowingly stepped from the adjacentconcrete walkway onto the “diamond plate”. The unexpected change in slip resistance caused theleading foot of the plaintiff to slip and the plaintiff was caused to fall. .
The subject location was examined by the slip resistance tester was used to measure the slip
resistance of the manhole cover surface. Six measurements were taken under dry conditions and six
measurements were taken after the surface was wetted. The average slip resistance of the manhole
cover was 0.75 when dry and 0.25 when wet. The slip resistance of the adjacent concrete was 0.74
when wet. Statistical analysis of the slip resistance measurements indicated that the average slip
resistance when wet was significantly smaller than the average slip resistance when dry.
Additionally, the slip resistance of the manhole cover was significantly smaller than the adjacentconcrete when wet.
The use of a slip resistant treatment for the “diamond plate” was recommended as an economical
method to increase the wet slip resistance and remediate the hazardous condition.

Case 3: Uncontrolled Environmental Factors

The plaintiff was spending the day at the beach before she returned to the Beach Bar to clean off.
The Beach Bar had a boardwalk surface leading from the beach to an outdoor shower area and thenonto a bar and the hotel. As the plaintiff approached the shower area, the boardwalk surfacechanged from dry to wet. The plaintiff’s foot slipped causing her to fall and sustain a wrist fracture.

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Testimony from the plaintiff and witnesses indicated that there were no mats, slip resistanttread material or other devices to provide slip resistance. In addition, there were no warning signs
in the area to warn that the boardwalk was slippery in the area of the shower.
Inspection of the location indicated that the boardwalk consisted of 2x6 timbers withnominal separation to allow drainage and expansion. The slip resistance in the area of the showers
was measured using the English XL VIT. The area was cleaned and dusted to remove
contaminants. The slip resistance was measured as 0.72 when dry and 0.55 when wetted withbottled spring water. An adjacent area was then tested after applying a thin layer of sand. Withsand present, the slip resistance was 0.32 when dry and 0.26 when wetted. The testing identified
that the boardwalk decking was slip resistant when the only contaminant was water. When sandwas introduced either on the patron’s foot or by other’s having showered, the problem significantly
increases to a hazardous state.
During the inspection, the work areas for employees were examined. The employer had
placed mats on the work areas of the boardwalk to lift the employee feet off the boardwalk andallow the sand and water to drain away.
Based upon the examination, the plaintiff’s human factors investigator opined that the third(Guard) and fourth (Warn) principles of that Accident Analysis Model were applicable. Due to the
outdoor location and purpose of the shower area, the hazard could not be eliminated by design nor
could the hazard be removed. As indicated by the facility’s use of mats under the employee workarea, the employer knew of the hazard and guarded his employees from the hazard. The patrons
were left exposed to the hazard. In addition to mats, handholds and/or railing should have been
installed near the shower area. As reported in testimony by witnesses, there were no warnings
posted to inform the plaintiff of the hazardous condition. A warning would have enabled theplaintiff to make a knowledgeable decision about whether and how to proceed on the walkway.

Case 4: Adequate Slip Resistance

The plaintiff was a guest at a local casino hotel. The plaintiff had spent the night in a guest room.
She awoke in the morning to use the toilet. As she walked from the toilet into the bathroom area,
the plaintiff fell to the ground and sustained a proximal fracture of the left humerus. The plaintiff
alleges that the floor was slippery and caused her to fall.
Casino-hotel employees are trained on proper cleaning techniques. Supervisors conduct
random written inspections and daily walk-in inspections of housekeeper performance. One written
random inspection is performed each week for each housekeeper. The floor was cleaned with adisinfectant and then with a hydrogen peroxide solution.
The plaintiff testified that she did not know if there was any water on the floor at the time of herincident. The plaintiff’s testimony indicated that neither the sink nor the shower had been used thatmorning and that the shower was not used on the previous night. The plaintiff was unsure whether
the toilet or the sink was used the previous night. The investigative reports characterized thebathroom floor as dry and clear. The plaintiff reportedly stated to security personnel at the time of
the incident that the floor was dry.
The incident location was examined and the slip resistance of the flooring was measured during
an inspection. The inspection indicated that the slip resistance of the floor in the area of the subject
incident, 0.73, exceeded the industry accepted standard of 0.50. The slip resistance measurements
indicated that the subject location was non-hazardous and would be considered slip resistant.
Based upon an analysis of the subject case, a site inspection, and the references cited above, it
was concluded to a reasonable degree of human factors and biomechanical certainty, that the subject
flooring did not pose a slipping hazard and the flooring was not the cause of the subject incident.

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Summary
The application of Human Factors principles can be used to determine the causal factors in slip andfall incidents. Slips and falls occur when there is a loss of balance as a result of too little friction
between the foot and walking surface. The Coefficient of Friction (COF) can be measured using
instruments such as the Variable Incident Tribometer (VIT). Data from the VIT method has been
accepted as evidence by United States Federal Courts and State Superior Courts.. The Accident
Analysis Model identifies the steps that property owners and business managers should use toensure their property is safe and employees and patrons are protected. The Accident Analysis
Model uses five major elements including Design, Remove, Guard, Warn, and Train. Three cases
were reviewed where the VIT and the Accident Analysis Model were used by the author in aforensic capacity.

References
Askren W & Howard, J. (2004). Road Map for the Practice of Forensic Human Factors and
Ergonomics. In Y. Noy & W. Karwowski (eds.), Handbook of Human Factors in Litigation(Chapter 5, pgs 5-1 – 5-16), Boca Raton: CRC Press.
Brungraber, R. & Templer, J. (1991), Controlled Slip Resistance. Progressive Architecture, 3, 112116.
Johnson, D. (2004). Measurement in Pedestrian Falls. In Y. Noy & W. Karwowski (eds.),
Handbook of Human Factors in Litigation (Chapter 20, pgs 20-1 – 20-28), Boca Raton: CRC
Press.
National Safety Council (2010). Protecting Ourselves from Slips, Trips and Falls, Retrieved
February 12, 2011, from http://www.nsc.org/safety_home/Resources/Pages/Falls.aspx.
Sacher, A., (1993). Slip Resistance and the James Machine of 0.5 Static Coefficient of Friction –Sine Qua Non, ASTM Standardization News, August, 1993.

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