Sent August 13, 2002

What Is a Safe Walking Surface?

Question of the Month
I have been asked this repeatedly. This question is also on the certification exam. The answer is "Any floor that has a slip index of .50 or better under its use conditions." And of course, the right answer on the exam is that it is any floor having a slip index of .50 or better measured with an English XL VIT.

There has been little controversy about the .50 threshold for clean dry surfaces. [See for a discussion of the validity of the .50 threshold.] In fact most of the traditional testing standards for slipmeters have specified means of cleaning and preparing the floor before testing. There are two problems with this approach:

1. Clean dry floors are not slippery under rubber shoe bottoms. And since the vast majority of shoes sold in the US have some kind of rubber heels, there is no point in measuring traction on clean dry floors.

Somebody says "What about the incredibly hard plastic topping lifts (the walking surface of a heel) that measure well below the threshold of safety on smooth hard floors?" There are two answers to that. (a) If a floor is reasonably safe for routine walking by the mainstream of shoes, any that are sold with heels that are excessively slippery are unsafe for their intended use. That is a liability problem for the shoe seller and not the proprietor of the floor. And (b) women in very high heels modify their gait so as to require less available traction for normal walking. They are in effect walking on their toes, and they take shorter strides.

2. Since there are few enterprises out there maintaining floors in the hyper-clean, dry conditions specified for the test, the test is not valid for evaluation of the safety of floors under real-world walking conditions. Any slipmeter that can't evaluate surface traction properties under actual conditions is irrelevant to pedestrian safety.

Almost anything on the walking surface will affect its traction performance. Water is the commonest contaminant occurring on floors to cause slip/falls. Water is the hardest lubricant to meter (as explained in the discussions of "sticktion.") [See for a discussion of sticktion and related terms. See for a brief discussion of the accredited pedestrian traction testing standards that are on the books.]

A Related Question
I am often asked "What slip index do you have to have on a dry surface for it to be safe when it is wet?" There is no valid answer to this question. If you want to find out what a surface's wet traction performance is, you have to test it wet. I recently tested a batch of 12 vinyl flooring samples for a manufacturer, and my results are instructive to this point. There was a wide range of performance among the 12 pieces, but one that averaged .94 dry turned out to average .17 when wet. And CXLTs know that .17 is the result you get on ice that is wet with water on the surface. Four other pieces that tested off-scale high under dry conditions gave wet indications of .18, .21 .65 and .85.

You can't make valid inferences by drawing invalid conclusions from partial data. There are surfaces that actually meter higher wet than dry, in rare instances.

In testing these vinyl samples I think I discovered evidence of why vinyl is not a good material for slipmeter workshops. We have known for years that vinyl gives inconsistent results, but for the first time I could actually see how the little bit of friction from the testfoot sliding across the surfaces changed the surface properties. I could even photograph the mark left by the sliding testfoot.

We have observed the same phenomenon on painted steel samples, and in our OSHA report we published the changes in traction performance we measured on the five test panels used by the ironworkers in their walk tests. That's why we use ceramic tile mostly in our workshops. It is hard enough and durable enough not to be significantly changed by the effects of the sliding testfoot. I have observed deposition of a film of Neolite on a ceramic tile that was repeatedly tested in the same spot, but we could not measure a change in traction performance in the case where we looked for a change.

These are all reasons why you need to move your meter around when testing floors so as not to take repeated readings on the same spot.

Another cautionary note for testing of vinyl samples--you need to wash the samples well, scrubbing them with a brush, to remove the temporary factory coatings. These affect traction performance significantly, and they are always removed by the first cleaning, so if you don't get rid of them to start with, you're not testing in the real world. Allow sufficient time (I allow overnight) to air dry before testing.

If the samples have ID tags on the back, you need to record this information before cleaning, because they are likely to be defaced or disappear during the scrubbing process. I always write the ID information on the top surface with a felt-tip marker anyway. It allows me to photograph all of the pieces with correct nomenclature, and it helps me keep my place on directional variations during testing. I used the Palmolive green soap in warm water for precleaning. I soaked the samples first for 20 minutes, scrubbed them, and then rinsed them with cold running water before drying overnught.

On Testfoot Contamination
In cases where certain operators' results were out of the ballpark [a little US baseball lingo there] in workshops and certification classes, it is almost always a contaminated testfoot that is the culprit. When a clean foot is substituted, the outlier almost always comes into the ballpark immediately.

If you use your meter on a surface having some sort of organic contaminant, it should be cleaned thoroughly before the next test exercise. Usually a good liquid soap like Palmolive "Original" green will do the job. Allow the foot to dry thoroughly before using it for testing.

XL users Page Eskridge and Charlie Turnbow tell me that they each have a certain test tile that they use periodically to verify that their feet are not contaminated.

The test tiles that C21 had obtained for reference use have been exhausted, but it doesn't have to be any standard tile. You can use any smooth, glazed ceramic tile, marble or terrazzo tile. Meter it dry and wet with a known pristine testfoot, and record the numbers on the back of the tile. You can then do a check periodically to make sure your foot is not contaminated.

William English, Inc.
Phone 239/728-3254, FAX 239/728-2304.
Visit for the latest in Slip Resistance Technology.

Copyright 2002 William English. This page may be forwarded freely if not altered in any way, but reproduction without the written permission of William English is prohibited.

| Go to the XL Slipmeter Page | Return to my Home Page |

| Return to Newsletter Archives |