The E-274 Trailer - Some Thoughts

The E-274 Trailer has been and still is a centerpiece of the Skid Resistance testing process on our nation’s highways. The basics of its design date back well over 50 years. I think this shows how solid the concept was when it was created and how well the manufacturers have maintained its essential qualities. Since Skid Resistance is such a complex metric, it is essential that we test it in manner that is as similar as possible to vehicles that rely on it. The E-274 test system comes closest to accomplishing that goal. While it does give us a very good indication of the Skid Resistance property, as we get more focused on this very important metric of our pavements, the trailer’s limitations are becoming more apparent. We now see the need for additional data in curves and near intersections. This requirement necessitates a higher sample rate than the traditional E-274 design can accommodate. Recently, this weakness has been addressed by updating the mechanical, control, and electronic systems enabling a much shorter and more rapid test sequence. A rate of twenty tests per mile is now readily achievable on a network basis and much higher test rates are possible for shorter sections. Resource limitations (water, tire wear, compressed air, etc.) now control the effective rate for network testing. The future integration of highway system data into the Skid Truck control system will allow variable rate sampling based on the network geometry. Another concern for the E-274 design is the measurement of the non-longitudinal horizontal forces on the test tire/pavement interface which are currently unmeasured yet may affect the values collected. While these forces are most probably not consequential when measuring a straight section, they could become significant when measuring around a sharp curve. Resolving this issue will require a redesign of the trailer so that the transverse forces can be measured. I anticipate that the equipment providers will be addressing this concern in the near future as highway safety moves ever so slowly from good rhetoric to actual practice.

John Andrews