High Friction Surface Treatment (HFST): How It Works

A high friction surface treatment, or HFST, is a road safety fix that you can spot by its color and texture: a rough, often tan or reddish layer laid across a sharp curve or an off-ramp. Underneath is a resin binder holding a coat of very hard aggregate that keeps tires gripping even when wet. It is meant for short, high-risk spots, not whole roads. Here is what it is made of, where it goes and how crews install it.

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The short version

What it is: per FHWA, a polymer resin binder holding a hard 1 to 3 mm aggregate that resists polish and wear onto the road surface.
The aggregate: calcined bauxite, required by current AASHTO and most state specs.
Where: tight curves, ramps, intersection approaches, steep grades and other spots where vehicles need extra friction.
Lifespan: at least 5 to 7 years under most conditions when installed properly, according to FHWA.

Why ordinary pavement loses grip

Skid resistance comes from two scales of texture. Microtexture is the fine grit on each stone; macrotexture is the bigger roughness between stones that lets water escape from under the tire. Traffic gradually polishes softer stone smooth, and water fills in fine texture. Our page on polished aggregate shows what that looks like. On a straight road the loss may not matter much. On a curve or a ramp where drivers brake and turn at the same time, it can be the difference between staying in the lane and leaving it.

HFST attacks both scales at once. The calcined bauxite resists polishing, so microtexture lasts, and the broadcast layer leaves a coarse finished surface for macrotexture.

Where agencies use HFST

FHWA's quick reference lists these primary applications:

Horizontal curves with radii under about 1,500 feet, or curves without enough superelevation for the speeds driven.
Curved ramps under about 1,500 feet radius, or ramps ending at intersections.
Approaches to intersections, especially with limited stopping sight distance.
Steep grades over about 10 percent.
Other high-risk spots such as pedestrian crossings and tunnels.

On a curve, the treatment usually starts where drivers begin to brake and ends at the point where the curve straightens out.

What the safety data shows

FHWA reports that HFST is most effective against run-off-road and wet-weather crashes. Its quick reference includes crash modification factors from a curve study: about 0.43 for total crashes, 0.28 for run-off-road crashes and 0.17 for wet-road crashes. A factor of 0.43 means the treated curves saw well under half the crashes that would otherwise have been expected. FHWA also cites benefit/cost ratios as high as 18 for the treatment.

Binder options

Epoxy resin: the most widely used. Strong and well bonded to asphalt and concrete, but it needs a dry surface and cures slowly in cool weather.
Polyester resin: strong, with good UV stability, and it can be catalyzed to cure faster when it is cool.
Methyl methacrylate (MMA): gains strength quickly in cool temperatures, though typically lower in strength than the others.
Polyurethane: stays a bit flexible and can be installed cooler, but is sensitive to moisture during installation.
Rosin-ester: a thermoplastic pre-coated on the aggregate and melted on site. It is ready for traffic once it cools.

How HFST is installed

Check the site. FHWA's checklist rules out deteriorated pavement, open-graded friction courses and surfaces with significant rutting, raveling or bleeding until they are repaired. New asphalt should be at least 30 days old.
Prepare the surface. It must be clean, dry and free of dust, with enough texture for a mechanical bond. Concrete is shotblasted. Pavement markings are removed or masked as needed.
Mix and spread the binder. Components are proportioned and mixed per the maker's directions for the day's temperature. Binder thickness is nominally half the largest aggregate size, and rougher surfaces take more.
Broadcast the aggregate right away, before the binder starts to gel, until no wet binder shows. The target is about 50 percent embedment.
Cure. The resin must fully harden with no soft spots before traffic is allowed on.
Clean up and reopen. Sweep loose aggregate off the lane and shoulders, replace striping and open the road.

Automated trucks that meter, mix and spread the binder are standard for larger jobs. FHWA's checklist suggests limiting fully manual application to small spots, typically 200 square yards or less.

HFST vs other surfaces

A chip seal also broadcasts stone onto a binder, but uses local aggregate and asphalt emulsion and covers long stretches for preservation, not targeted friction. An open-graded friction course improves wet-weather driving by draining water, but it is not a candidate for HFST on top. HFST often works alongside other low-cost safety measures such as rumble strips and high-visibility thermoplastic markings. It is rarely used on private driveways; a steep, slick drive usually needs a new textured surface rather than a highway safety product.

Questions people ask

What is a high friction surface treatment?

It is a thin layer of very hard, polish-resistant aggregate, about 1 to 3 mm in size, glued to the pavement with a polymer resin binder. FHWA describes it as a spot safety treatment that raises friction where vehicles need it most, such as sharp curves and ramps.

What aggregate is used in HFST?

Calcined bauxite. FHWA's quick reference notes it has a proven record for both initial and long-term friction and is what current AASHTO and most state specs require.

What binders are used in HFST?

Epoxy resin is the most common. Polyester, methyl methacrylate, polyurethane and rosin-ester systems are also used. They differ in cure time, cold-weather behavior and moisture sensitivity.

How long does HFST last?

FHWA says a properly installed treatment should last at least 5 to 7 years under most conditions, and longer with lighter traffic or on newer pavement.

Does HFST reduce crashes?

FHWA's quick reference cites a curve study with crash modification factors of about 0.43 for total crashes and 0.17 for wet-road crashes, meaning far fewer crashes than expected at the treated curves.

Can HFST go on any pavement?

No. FHWA's checklist says deteriorated pavement, open-graded friction courses and surfaces with significant rutting, raveling or bleeding are poor candidates until repaired, and new asphalt should be at least 30 days old.

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