Consultant's Corner: Are Your Driveways and Walkways Safe?
By: John C. Fairchild, P.E.
More than one half of a million people living in
Walking and driving surfaces often warrant concern due to slippery conditions. Perhaps a new traffic deck coating that was applied to a parking garage to provide protection of the structure against water infiltration became a major liability issue when vehicles and pedestrians started slipping on the newly applied coating. Or, what if integrated architectural concrete, granite and marble walkways in your building become very slick when wet, causing falls and injuries to became all too frequent. When situations like these arise, as they sometimes do, owners have to take prompt action.
Although most people make quick assessments of what they personally deem to be slippery, there is no real consensus as to defining the limits of acceptable slip resistance, particularly when considering wet surfaces. In an effort to quantify slip resistance and develop guidelines for safe walking and driving surfaces, theoretical analyses and empirical research have established the coefficient of friction for various surfaces, ranging from 0 for no slip resistance to 1.0 for 100 percent slip resistance. Although many experts have offered opinions as to what defines safe walking and driving surfaces, building codes and other governing authorities have typically provided minimal, simplistic, and/or conflicting guidance on establishing what is safe versus what is unsafe.
The current International Building Code (IBC) provides no requirement for slip resistance, other than to specify “slip resistant surfaces.” The Occupational Safety and Health Administration (OSHA) and the Federal Trade Commission (FTC) reportedly recognize 0.5 as the minimum requirement for slip resistance, but the Americans with Disabilities Act Accessibility Guidelines (ADAAG) only require that accessible routes be “stable, firm and slip resistant.” While not providing standards or methods of measurement for the definition of “slip resistance,” Appendix Section A4.5.1 of the ADAAG does, however, recommend that walking surfaces have a “static coefficient of friction” of 0.6 for horizontal accessible routes and 0.8 for ramps.
Intuitively, we understand that wet surfaces are more slippery than dry surfaces. We also recognize that slipping when we are moving is more likely than when we are standing still. Unfortunately, traditional methods of quantifying the static coefficient of friction do not take into account wet surface conditions or dynamic motion, both of which research has found to be critical in slip/fall incidents. In addition, research has also indicated that unsafe walking and driving surfaces are created when significant variability in slip resistance exists within contiguous walking and driving surfaces and/or when pedestrians or drivers are not aware of hazardous conditions. For example, research has shown that people can walk safely on ice when they know it is ice, but invariably fall if they unexpectedly encounter an icy surface. How then can you evaluate the safety of your walkways and driveways?
William English, SCP, PE, a well-respected expert in the field of tribometry (the science of the measurement of friction/traction), has researched slip/fall issues for a number of years and has developed a testing apparatus – the English XT VIT (variable incidence tribometer) that can be used as an important tool in the evaluation of the slip resistance of walking surfaces. He has also offered recommendations for standards in evaluating and improving the slip resistance of walking surfaces. Although the slip resistance measured by the English XT VIT is roughly equivalent to the “static coefficient of friction” referenced in the ADAAG for dry surfaces, it is also able to take into account slip resistance under dynamic forces on wet surfaces. In utilizing his instrument, English has established a “slip resistance” of 0.5 as the threshold of safety on wet surfaces.
If your facility has areas perceived to be slippery, the first step toward resolution is to ensure that normal cleaning operations are not capable of improving the areas. If normal cleaning operations alone are not effective, improvement of deficient areas may require a formal engineering evaluation. While it may not be possible to eliminate all potential slip/fall hazards at your facility, it should be possible to minimize such hazards by combining objective testing with common sense. On the basis of our experience, some common solutions to slip resistance problems include the following:
1. Use conspicuous signage to notify vehicular and pedestrian traffic of potential slip hazards.
2. Pressure wash to improve slip resistance for a variety of surfaces, including aggregate-embedded traffic deck coatings. If pressure washing alone is not successful, the use of mechanical or chemical means to expose edges of the aggregate may also be effective. It is important, however, to ensure that such techniques do not damage the substrate or coating.
3. Apply proprietary cleaning/coating products to improve the slip resistance of suspicious surfaces.
4. As a final resort, replace unacceptable walking and driving surfaces with ones that historically have provided acceptable slip resistance. If the suspicious surface is a traffic deck coating, the use of a different size or type of aggregate or the application of a different type of coating may be effective in correcting persistent slip issues.
Slippery surfaces do not have to be accepted as a normal, unavoidable condition. While many slip issues can be corrected by incorporating routine cleaning and maintenance, chronic problems may need a formal engineering evaluation. By utilizing objective testing and common sense, improvements are possible for most surfaces. In any case, the goal should be to address and correct, not to ignore, slippery walking and driving surfaces.
John C. Fairchild earned a BS in Architectural Engineering and an MS in Civil (Structural) Engineering from the