Anti-Skid Surfacing

What is anti-skid surfacing?

Anti-skid surfacing is a specialist road safety treatment that bonds a coarse, high-polish-resistant aggregate to the road surface with a resin or thermoplastic binder, raising the wet skid resistance of the finished surface above the Investigatory Level set for the site.

The technique is also called high friction surfacing or HFS, and appears in the UK industry under several related names: anti-skid road surfacing, skid resistant surfacing, anti-slip road surfacing, and high friction surface treatment. All refer to the same technique. The aggregate is almost always calcined bauxite, a heat-treated aluminium ore prized for its hardness and its resistance to polishing under traffic. The finished surface is deliberately coarse, creating many high-contact-pressure points under a tyre, which is what produces the grip.

Anti-skid surfacing is specified for high-risk locations on UK roads: approaches to junctions, traffic light stops, pedestrian crossings, roundabouts, sharp bends, motorway slip roads, and other points where sudden braking is expected. It is applied as a thin overlay to an existing road surface, not as a full-depth construction. Local authorities and National Highways both use it, and the Water UK and NFCC guidance aside, the governing standards in the UK are SHW Clause 924, the RSTA ADEPT Code of Practice, and the BBA HAPAS scheme.

McFadden Utilities delivers anti-skid surfacing within two service contexts: as part of a Tarmac and Road Surfacing scheme where HFS is specified upfront, and as part of Reinstatement Works where a utility opening has disturbed an existing high friction surface and triggered the SROH S6.4.12 reinstatement duty.

How anti-skid surfacing works

Anti-skid surfacing works by fixing a very hard, angular aggregate to the road surface in a thin bonded layer. The aggregate creates a dense pattern of high contact pressure points under a vehicle tyre. Under braking, those points resist polishing and cut into the tyre tread slightly, raising the coefficient of friction between tyre and road. The effect is strongest in wet conditions, where standard asphalt loses grip but anti-skid surfacing retains it.

Calcined bauxite is the aggregate of choice. It is produced by calcining (heat-treating) bauxite ore, which is the same ore used to make aluminium. Calcination drives off the water content and converts the ore into a hard, dense synthetic aggregate. UK suppliers such as LKAB Minerals source from China or Guyana. Calcined bauxite has a high Polished Stone Value (PSV) and a high resistance to abrasion, which is why it holds its texture under traffic where a natural aggregate would polish smooth within a year or two.

Two application methods are used in the UK: hot-applied and cold-applied. They produce the same end result but reach it differently.

Hot-applied systems

Hot-applied anti-skid surfacing uses a thermoplastic binder that is pre-mixed with calcined bauxite and heated on site in a boiler. The hot mixture is applied to the road using a screed box, which meters the material at a controlled thickness as it is drawn across the surface. The coarse texture forms as the mixture cools and sets. Hot-applied systems typically deliver a service life of three to five years per the RSTA Code of Practice, with a mid-point of four years. Installation is fast once the boiler is up to temperature, and the surface is trafficable within hours of application.

Cold-applied systems

Cold-applied anti-skid surfacing uses a two-part resin, typically epoxy, polyurethane, or methyl methacrylate (MMA), applied to the road surface first. Calcined bauxite aggregate is broadcast over the wet resin, which then cures at ambient temperature. Excess aggregate is swept up once the resin has set. Cold-applied systems deliver longer service life, typically five to eleven years per the RSTA Code of Practice, with a mid-point of eight years. MMA systems cure faster than epoxy or polyurethane and can allow the road to reopen within an hour of application in suitable conditions.

When anti-skid surfacing is specified

Anti-skid surfacing is specified where the combination of site geometry, traffic volume, and collision history indicates that standard surface course skid resistance is not enough. In the UK, the decision follows from the Investigatory Level framework set out in DMRB CS 228, which assigns an IL to every section of the strategic road network and, by extension, to sections of local authority highway.

Typical locations where anti-skid surfacing is specified include:

• Junction and traffic light approaches. Heavy, abrupt braking is expected. The treatment length covers the braking distance up to the stop line.
• Roundabouts. Applied at the entry taper and sometimes on the circulatory carriageway where entry speeds are high.
• Pedestrian crossings. Both zebra and pelican approaches, to shorten stopping distances.
• Sharp bends and steep gradients. Particularly where a loss-of-control collision record exists or where road geometry is below current design standard.
• Motorway slip roads and emergency areas. Transitions between high-speed and low-speed operation, including emergency bays on smart motorways.
• Bus and cycle lanes. Often in a coloured finish (red for bus lanes, green for cycle lanes) to reinforce the lane designation visually as well as raising grip.

Coloured applications sit inside the same specification framework. The aggregate is the same calcined bauxite, tinted or overlaid depending on the system. Natural buff calcined bauxite is also used where colour is not required but visual differentiation from the surrounding carriageway is helpful.

Terminology

UK standards and statutory framework

The specification chain

The UK specification framework for anti-skid surfacing runs through four documents, working from the top-level specification down to the industry code of practice:

On the materials side, BBA HAPAS defines a high friction surface as having a minimum Skid Resistance Value of 65, measured using the portable Skid-Resistance Tester as defined in Road Note 27. BBA HAPAS certificates are valid for three years unless withdrawn. Only products and installers on the HAPAS register may be used where the specification calls for a HAPAS-certified system.

On the design side, Investigatory Levels set in CS 228 trigger the requirement. Local authorities set their own skid resistance policies that translate CS 228 into council-specific thresholds, and National Highways applies CD 236 and CS 228 directly on the strategic road network. The RSTA Code of Practice is peer-reviewed by ADEPT (Association of Directors of Environment, Economy, Planning and Transport) and endorsed by the Road Safety Marking Association.

Reinstatement under SROH

Anti-skid surfacing creates a statutory duty for any contractor who excavates through it. Under the HAUC Specification for the Reinstatement of Openings in Highways (SROH), clause S6.4.12, which came into force as statutory guidance on 10 May 2021, high friction surfaces must be permanently reinstated with a like material (or an agreed alternative) within 15 working days of the completion of the reinstatement.

The statutory basis sits in Section 71 of the New Roads and Street Works Act 1991, which gives the Secretary of State power to set reinstatement specifications for utility openings. SROH is the code of practice produced under that power. The reinstatement material must hold a Product Acceptance Scheme certificate (BBA HAPAS for HFS), and must be laid in accordance with the manufacturer’s instructions by a contractor with suitable experience. In practice, this means the utility contractor who opened the road is responsible for the reinstatement scope, and engages a HAPAS-approved specialist installer to apply the HFS overlay itself.

The 15-working-day window can be extended if weather or site conditions justify delay (for example, if the new surface course underneath needs to be trafficked or aged before a cold-applied HFS system can be applied). Interim warning signs indicating a potentially slippery surface are required until the permanent reinstatement is complete.