1. What is a High Friction Surface Treatment (HFST)?

Definition

• A High Friction Surface Treatment is a cost-effective safety countermeasure in which a polish-resistant aggregate such as calcined (i.e., heat treated) bauxite aggregate is bonded to the pavement surface using a polymer resin binder, significantly enhancing skid resistance and reducing crashes.

Description

• HFST places a layer of highly durable, anti-abrasion and polish-resistant aggregate over a thermosetting polymer resin binder. The mineralogical and physical properties of these aggregates make the overlay exceptionally resistant to wear and polishing by traffic. The polymer resin binder locks the aggregate firmly in place, creating an extremely durable surface capable of withstanding even the most extreme roadway demands, from cornering and heavy braking to snowplowing. HFST restores, and, in most cases, significantly enhances, pavement surface friction where traffic has worn down existing pavement surface aggregates. HFST can also help compensate for inadequate geometric designs, such as sharp curves and substandard superelevations.
• Aggregates used for HFST, by definition, have a high polished stone value (PSV). Please see question #11 under the Material Specifications/Durability section for more details on PSV. Laboratory PSVs have been correlated with successful HFST performance in field installations. Although several aggregates have been evaluated, only calcined bauxite aggregate has consistently met the threshold for performance necessary to be used for HFST.

2. What is the purpose of HFST?

• HFST can enhance the ability of a road surface to provide adequate pavement friction for vehicles in critical braking or cornering maneuvers. Maintaining the appropriate amount of pavement friction is critical for safe driving. Compared to tangent sections of road, horizontal curves and intersections have a much greater friction demand for keeping vehicles on the road and ensuring safe stopping distance, particularly in wet weather. In locations such as sharp horizontal curves, where vehicles apply higher side-shear force to the pavement, the pavement surface tends to polish faster, reducing the available pavement friction. Reduced friction can contribute to vehicles losing control or skidding when they are traveling at excessive speed, make abrupt turns, or brake excessively. Higher friction, maintained with polish-resistant aggregates, helps to keep vehicles on track.

3. Where is HFST likely to be beneficial?

• HFST technology has benefited State transportation agencies, counties, cities, Tribes, and Federal agencies across the country. The greatest benefit from HFST is often gained where friction demand exceeds the available friction of a roadway surface. These areas are often interchange ramps, rural curves, steep grades, and intersections. HFST has also been successful when used in lieu of costly roadway realignments, which can take years to complete due to delays associated with lengthy planning phases and budget constraints.
• While HFST has been proven successful in preventing crashes under dry roadway conditions, installing HFST where clusters of wet weather crashes have occurred may demonstrate the greatest benefit.
• Some agencies have made a determination to install HFST systemically as a preventive safety countermeasure based on specific roadway characteristics and friction at various locations. HFST can increase roadway friction significantly in locations with these common characteristics, thus increasing roadway safety.

4. Are there guidelines for HFST installations limits?

• Typically, HFST should be installed at a point where vehicles start to brake. At horizontal curves, brake lights are a good indication of where treatment should start. Most States will end the treatment at the point of tangency (PT).
• The Texas A&M Transportation Institute published a paper titled, "Evaluating the Need for Surface Treatments to Reduce Crash Frequency on Horizontal Curves" that provides recommendations on how to select start and end points for HFST installation. This paper can be found at: https://static.tti.tamu.edu/tti.tamu.edu/documents/0-6714-1.pdf

5. What is the state of HFST installations around the nation?

• The FHWA Office of Safety web portal provides a map for the current status of HFST implementation at: https://safety.fhwa.dot.gov/roadway_dept/pavement_friction/high_friction/