Maintaining the appropriate amount of pavement friction is critical for safe driving. In locations where drivers may brake excessively; for example, when going around curves, down hills or steep grades, or when approaching an intersection; the road surface can become prematurely polished, reducing the pavement friction and allowing vehicles to skid when the drivers brake. Drivers may also be speeding or distracted, contributing to the high-crash rates in critical locations. Wet road surfaces can also reduce pavement friction and cause skidding or hydroplaning.
Critical locations make up a small percentage of U.S. highways. In 2008 for example, horizontal curves made up only 5 percent of our Nation’s highway miles. Yet, more than 25 percent of fatal crashes occurred on horizontal curves. High friction surface treatment (HFST) is an emerging technology that dramatically and immediately reduces crashes and the related injuries and fatalities. With friction demands far exceeding conventional pavement friction, high-quality aggregate is applied to existing or potential high-crash areas to help motorists maintain better control in dry and wet driving conditions.
While the initial costs are higher than conventional pavement, however, the long-lasting durability of HFSTand limited use in critical locations makes the product a low-cost option over its life cycle. HFST may also be used to identify specific areas, such as bus or bike lanes, or used on surfaces that tend to ice such as bridges or pedestrian walkways.
Several high friction surface treatment products are available now and other, more cost-effective products are being developed. The HFST products use aggregates that are both polish- and wear-resistant and develop channels to prevent water buildup on wet surfaces. The bonding materials such as Epoxy and other available blends are designed to set quickly. HFST can be applied by machine at a similar speed to other paving surface treatments, or applied with hand tools, but the road surface must be durable with few to no cracks and crumbling.
Motorists may notice rougher riding surfaces in treated areas; however, they also will experience greater pavement friction resulting in better control of their vehicles. Friction improvement projects have been well received by the public and elected officials because the results are measurable, the costs are relatively low, and the products produce negligible environmental impacts.
Additional analysis to develop crash modification factors for these types of treatments is underway as part of FHWA’s Evaluation of Low-Cost Safety Improvements Pooled-Fund Study. The Office of Safety has included HFST in the countermeasures recommended on curves when developing Safety Implementation Plans in Roadway Departure Focus States.