EQUIPMENT for collecting PAVEMENT SKID RESISTANCE

Pavement Skid Resistance is measured by equipment utilizing both stationary and traveling processes. The stationary processes typically sample a very small patch of the surface and therefore focus primarily on micro-texture. An exception to this limitation are the laser based systems that measure the surface contour. Due to speed, spot size and sensor size limitations, the laser based devices are currently limited to macro-texture but may well get down to micro-texture resolution in the near future when applied in stationary mode and in several years when applied in traveling mode.

The most popular traveling equipment currently used to collect pavement Skid Resistance in the United States is based on the ASTM E-274 standard and typically is used in LOCKED ROTOR mode. There are a number of other devices utilized, especially for airport runway evaluation that create a different challenge to the pavement/tire interface and measure the forces involved. A number of programs and research efforts have attempted to harmonize these different devices and the E-274 device over multiple pavement surfaces with little overall success. While the values delivered are typically proportional, their relationships vary significantly when applied to multiple pavement types. The measurement of pavement Skid Resistance is clearly a work-in-progress.

Clearly the preferred method to measure the pavement’s contribution to Skid Resistance would be to measure directly the applicable range of pavement contour and relate it to the Skid Resistance provided to a typical vehicle on the highway. This process will need to be developed after a reliable and accurate method of measuring micro-texture on a large scale is developed and the research is completed that relates the various aspects of pavement contour to skid resistance. This appears to be a decade or probably more in the future.

We are therefore left with measuring the Pavement Skid Resistance by applying a challenge to the tire/pavement interface in a controlled manner and preferably in a very similar fashion to the typical vehicle on the highway. When the E-274 standard was developed, anti-lock braking systems were not on the horizon much less deployed. Now, they are standard on the vast majority of vehicles travelling our roads. I believe we need to modify the E-274 system to measure the skid force prior to full lock-up. I have proposed averaging the measured force between 33% and 66% lock-up. We need to begin the measurement beyond 20% lock-up to minimize the impact of the construction of the measurement tire. At 40 mph, today’s equipment can typically collect 10 or more data points in that range and average them for a Skid Resistance value which I suggest we call SSN (Slip Skid Number). Several thousand miles of pavement have been collected in this fashion and the SSN was typically proportional to, and roughly 125% of the SN (locked rotor} value with an R squared above 0.99 across all pavements. However, on an individual pavement by pavement basis, the relationship varies by over 10%. I believe this difference reflects the fact that this test is impacted by slightly different aspects of the pavement contour. If we agree that this is the case, the SSN value should better relate the pavement’s performance to the needs of current vehicles. Other advantages of collecting the SSN is that it is a very rapid test and it that uses much less resources than the locked-rotor test. As often happens when new technology is employed, the difficulty will be in relating the SSN to SN values collected in the past. In addition, significant modification needs to be made to both the hardware and software of an E-274 device to collect and report the SSN, but the first versions have been produced and are working. Future improvements to the E-274 device should include a trailer mounted, computer controlled, water delivery system and an electromagnetic or hydraulic braking system eliminating the use of pads and rotor.

Another potential area for improvement in testing for Skid Resistance is adjustable test intervals based on the length of anticipated homogeneous pavement surface. Currently, the equipment tests on a fixed interval (unless manually over-ridden) which is set at the start of the road to be tested, The sections at ramps, curves, and intersections can have quite different SN or SSN values than the remainder of the pavement. Utilizing the network database, these sections can be identified and the information transferred with the test requirements. The appropriate test interval can be set and transferred to the collection equipment so that the test resources can be deployed in an optimum manner. Test intervals could be varied from every 0.3 miles for long straight stretches of pavement down to every 0.01 miles.for critical sections.

In summary, the available equipment for measuring Skid Resistance is functional, but there is much room for improvement and for new technological development. There is also much room for a better understanding of the traction challenges faced by today’s pavement users and how the various characteristics of the surface contour impact the resultant Skid Resistance.