Should the Threshold of Safety Be .50?
William English, CSP, P.E. (CA)
Last Revised April 30, 2006

This was originally written as a draft of a rationale for a threshold of safety for consideration by the ANSI A1264.2 Committee on slip resistance in the workplace and has been expanded since.

During his important pioneer work at the National Bureau of Standards in the WW II era, Percy Sigler observed that dry surfaces are not slippery, and since the articulated strut machines and dragsleds could not meter wet surfaces with validity, he set about to develop a slipmeter that could measure available traction on lubricated floors. Despite his very respectable studies, which were widely published, many tribometric devices have been subsequently invented, patented and marketed which cannot meter wet surfaces because of the sticktion phenomenon, which causes instruments with a residence time prior to actuation to give artificially high readings on wet surfaces so as to be misleading in the evaluation of slip resistance.

In the current decade slipmeters operating at velocities that are commensurate with those encountered in ambulation (and which can avoid sticktion by avoiding residence time) have become available, namely the PIAST and the VIT. For these devices to be useful in determining compliance with performance-based guidelines, a specific slip index that affords a reasonable measure of safety must be specified. In order to discuss what that number should be, we must look at historical investigations and guidelines.

Underwriters Laboratories
Meanwhile, Sidney James, working at Underwriters' Laboratories, modified the original dynamic friction tester Hunter had developed at NBS resulting in the massive static coefficient of friction machine that bears his name to this day. It became the standard for rating of floor polishes and other materials by UL criteria for slip resistance. Mr. James is said to have conducted experiments by walking on polished surfaces that were thought to be adequately slip resistant, taking increasingly long strides until his heel slipped. By performing the trigonometric calculations recognized at NBS and in physics texts as the formula for static coefficient of friction (as measured by the articulated strut principle), he came up with the .50 SCOF as the threshold of safety for normal mbulation. That index is still the basis for listing of floor finishes and other products as slip resistant, under UL 410.

ASTM D-21 on Floor Polish
This long-standing group has followed Sidney James' doctrine unswervingly and continues to support his .50 threshold of safety in connection with ASTM D-2047.

Construction Safety Regulations
for steel erection promulgated on January 18, 2001 specified minimum slip resistance performance for painted fabricated steel components of .50 as tested under wet conditions with either a Brungraber Mk II or an English XL by a competent laboratory. The specification is in 1926.754, (c), (3). The Department of Labor has said that our certification would be adequate proof of competence for compliance purposes. In a USDOL news release announcing these new rules, OSHA said:

This specification has since been withdrawn by OSHA, without explanation. There is also an OSHA spec for .50 in 1910.68, but no means of measuring is specified there.

In proposed rule making published in the Federal Register April 10, 1990, the Occupational Safety and Health Administration in the US Department of Labor specified a recommended value of .50 in workplaces where lubricating contaminants could make footing hazardous, without adequate surface treatment. OSHA's purpose is to require the improvement of safety by increasing floor traction. Of course those proposed emendations to the OSHA General Industry workplace safety regulations have never been promulgated, for political reasons; but they are still on the books as a pending proposal. And many knowledgeable observers agree that the .50 value could be legally enforced now under the "General Duty Clause" of the OSHAct. The cases where employers have been cited for slippery walking surfaces under the General Duty Clause have not been overturned on appeal.

NFPA 1901 Standard for Automotive Apparatus, 13-7.3 recognizes only the VIT and the PIAST for testing wet surfaces. For a nice explanation of the standard and discussion of means of complying, click here.

Access Board
At the ASTM F-13 symposium on Slips, Trips, Stumbles and Falls in Denver on October 23, 1989, Frank Buczek presented a paper describing work that he and his colleagues had performed to measure required traction for various individuals having mobility impairments in locomotion. Their findings were accepted by the Architectural and Transportation Barriers Compliance Board (ATBCB) as the basis for the thresholds of safety they subsequently specified for public access routes.

Values published in the Federal Register on July 26, 1991 as Americans with Disabilities Act Guidelines called for a slip index of .60 on level walkways and .80 for ramps up to a slope of 1:12.

ASTM C-21 on Ceramic Tile
In a letter ballot from C-21 bearing an issue date of June 20, 1996, a "New Standard Specification for Acceptable Coefficient of Friction of Ceramic Tile for a Walking Surface" (Z6334Z) specified a threshold of safety of .50

A1264.2 on Provision of Slip Resistance on Walking /Working Surfaces sets forth a .50 threshold of safety.

US Army Construction Spec
Beginning on p. 459 of 27.E.15, Walking/Working Surfaces, Section C, the .50 minimum performance (measured wet) on painted structural steel components has been adopted by the Army, apparently without the grandfather delay period specified by OSHA.

Case Law
For the past half century court precedent has allowed that .50 is the recognized threshold of safety, without serious challenge to this number.

Expert Opinion
Although different spokesmen for pedestrian safety have disagreed on what the specific number should be, most seem to be in approximate agreement among the experts. For example, Robert Brungraber, who has been recognized in print as the world's preeminent authority on pedestrian slip resistance as it relates to safety, has often averred in F-13 Committee meetings and elsewhere that .50 should be specified as the threshold of safety. In his book, Slip & Fall Practice, 2nd Ed, 1995, Charles E. Turnbow endorses the traditional 0.50 threshold of safety and recommends the English XL slipmeter for measurement. He rates a .40 measured with a Sigler pendulum tester as equivalent to .50 with the XL.

Another investigator of pedestrian safety, Don Meserlian, who recognizes himself in Steve Rosen's latest edition of the Slip and Fall Handbook as "one of the world's leading experts on pedestrian safety," is on record as supporting the .50 slip index as the threshold of safety in ambulation. Don claims to have replicated studies done by Ekkebus and Killey (as published by the CSMA in February of 1973), as have many others during the intervening years, confirming their statement that "To add a safety factor, the figure of .5 becomes valid." (p. 252)

English and Marletta published a report on May 20, 1995 in which we set forth the results of our research findings of traction requirements for pedestrian safety on painted steel surfaces (under a contract in connection with SENRAC). The consensus of the ironworkers who subjectively evaluated the surfaces by walking on them under wet conditions was that a slip index of .75 as measured by the English XL Variable Incidence Tribometer or .60 as measured by the Brungraber, Mk II (the PIAST) was adequate for safe walking at high elevations.

These men did not say that the figure they chose was a minimum value for safe walking, however. The consensus among them was something to the effect that "A value of .75 is easily achievable and feels real good under my feet when it is my life at stake at 100 feet altitude." In fact there were no marginal paints in our study. The smooth ones were patently hazardous when wet, and all of the textured paints passed. A threshold of .50 would have passed and rejected the same finishes. The complete SENRAC report and copies of documentation used by our opponents to oppose adoption of the recommended specification are contained in my book Pedestrian Slip Resistance as a 47-page appendix, complete with tables and photographs.

English, Underwood and Vidal Published a research report of a pedestrian slip resistance study performed for OSHA (under federal Purchase Order DC 20210) dated November 24, 1998 in which they recommended .50 as the threshold of safety on wet surfaces.

Plenty of Historical Precedent
The subject of minimal traction demand for safe ambulation has been well studied over the past 60 years or so. From the start a slip index of .50 has been presented as a minimal value for safe walking, allowing some margin of safety.

There has been no serious objection to the .50 threshold when it is applied to the safety of dry walking surfaces. But there has been heated controversy concerning the threshold of safety on contaminated or lubricated surfaces, although a pedestrian's traction requirement is always the same in normal ambulation, regardless of what he is walking on. However, there has been no significant research published in the scientific literature in modern times to seriously challenge the .50 threshold. That is, no recognized expert has testified or written convincingly establishing a lower figure, though higher ones have been proposed, as has been pointed out above.

Certain studies related to the athletic shoe industry have produced rationales for slip indices well in excess of 1.0, but that has no relevance to the traction demand for normal walking safety. The long-standing .50 is adequate for normal ambulation by normal people, with some margin of safety.

Lubricating Contaminants
Since our consideration in ANSI A1264.2 is primarily for workplace safety, and since many kinds of lubricating contaminants challenge the safety of employee footing in industrial processes, we should concern ourselves primarily with slip indices of contaminated surfaces. And since .50 is a largely unchallenged value for the general public on dry surfaces, the same traction value would afford reasonable safety for workers in industrial environments for many workplace activities on level surfaces where lubricating contaminants are present.

It seems certain that higher slip resistance would be required on sloped surfaces and where the work activities involve pushing or pulling or other physical exertion, but since we are only proposing a general guideline, it should be pointed out that professional judgment would be required as to what level of increased slip resistance may be prudent in those special situations.

In an interesting aside, certain proponents of the James Machine who allege that it is the benchmark against which all other tribometric means should be compared are caught in a Catch 22 situation, in that the James cannot be used on wet or otherwise lubricated surfaces with validity, so there can be no means of comparing valid metering principles to the (dry) James baseline.

Also the jurisdictional squabbling about who uses which instrument to measure traction properties of different materials seems inappropriate in our context in that we are talking workplace compliance and not general liability exposures; and none of the vested interests dominating various ASTM tribometric standards committees has addressed industrial safety, particularly under contaminated conditions. They are peculiarly concerned with avoidance of law suits in the public liability arena.

But the establishment of .50 for general exposure would enhance workplace safety significantly, and it would be within the scope of safety managers in industries where a higher threshold would be needed (in applications where greater physical exertion would dictate higher available traction) to determine what higher values would be professionally prudent. No guideline can contemplate every contingency, but our standard should specify a minimal level of performance that is widely supported by the body of scientific knowledge. A minimum slip index of .50 would be the least controversial number to choose for workplace exposures in our context.

Other Factors Affecting the Threshold of Safety

The required traction shown on the ambulation charts is not controversial. All of the GRF traces produced by ambulation labs over the past quarter century agree. Our critics often point out that the ambulation traces show a traction demand during walking of about .25 (as illustrated in the chart of Gronqvist et al as shown on page 31of Pedestrian Slip Resistance, Second Edition) as if that argument would overthrow our rationale for the .50 threshold.

The facility proprietor normally has little control over the footwear that invitees will wear there, so we can only expect him to be accountable for the traction properties of the floor. A slipmeter only measures the traction properties of the floor. Its output is not relevant to any particular shoe type except that the Neolite pad is within the range of shoe bottom materials commonly used. But the .50 target is a slip index for the floor as metered by the English XL Variable Incidence Tribometer. Further, the Neolite pad is specially prepared by cleaning and sanding (to allow repeatability). It will therefore give a higher traction number than would be derived under an actual shoe of the same material that had been worn for general walking and had general surface contamination of various types.

Therefore, because our slipmeter friction pad is artificially prepared so that it is pristine and uniform each time it is applied to the test surface, it is giving us readings that are a bit higher than would occur under most normal (soiled) conditions. And some shoe bottom materials are less slip resistant than Neolite, so if we are to have a safe level of traction for a wide range of walkers wearing various kinds of shoes, a margin of safety is required. The proponents of the .50 have said that it incorporates this margin of safety. They did not say that available traction of .25 is safe for walking. They were saying that the .50 threshold assures that most people would have adequate traction for normal walking. For more insights into this discussion, take another look at Ten Myths Concerning Slip Resistance Measurement.

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