Guidance of the Colour of High Friction Surfacing (HFS)


 

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High Friction Surfacing (HFS) is used extensively in UK, US, Ireland, New Zealand & Australia. An important component of HFS is Calcined Bauxite (CB), which has a high PSV, low AAV and is an exceptionally hard & durable angular aggregate. CB is generally available in two colours, Buff & Grey. 

 

Chinese Buff Bauxite                             Indian Grey Bauxite

 

Whereas the various specifications cover in great detail the installation & performance requirements, there is very little guidance on the most appropriate colour. However the recently released ARRB HFS good practice document from Australia recommends that the colour should be nominally grey unless otherwise specified. In other parts of the world, it generally seems to be a case of which colour the specifying engineer is used to.

The most common applications for HFS include:

  • Dangerous bends
  • Approaches to pedestrian crossings
  • Approaches to roundabouts & junctions
  • Steep inclines
  • Roundabouts with skidding difficulties
  • Other locations with a high risk of skidding

So what is the most appropriate colour to use for each application?

 With Grey, the driver will not be aware of the different surface and his behaviour will not be influenced by the surfacing. Road markings remain very visible. With Buff, the driver is aware of a change. Road markings are not as visible, particularly at night. There is a school of thought that some drivers ‘in the know’ adapt their driving behaviour & actually speed up when they realise there is a visible skid resistant surface on the road.

 For approaches to pedestrian crossings, I would suggest that Buff is better as it gives the driver warning of the significant hazard ahead. 

Buff HFS at Pedestrian Crossing Approach in Dublin

 

For dangerous bends, roundabouts & steep inclines I would suggest that Grey is better so that the driver’s behaviour is not influenced by the safety surface. 

Grey HFS at a Dangerous Bend

 For approaches to roundabouts & junctions, I think it should be site specific & a case of engineering judgement. Does the Engineer wish to highlight the change in surface to the driver or is it better if the driver is unaware of the safety surface?

Grey HFS at Junction in London

 Another point to consider is the aesthetics of when the HFS begins its gradual process of failure. We have all seen the situation of failing Buff HFS, which can look like a patchwork quilt of grey & buff. Recent research has indicated that HFS, which has started to fail, may provide Scrim values above the relevant Investigatory Level (IL) for many years, but nonetheless it is very unsightly. When Grey HFS begins the process of failure, it is not visibly very obvious due to the colour similarity to the asphalt road. The Engineer will be under less pressure to prematurely replace it.

Buff HFS in Failure Mode

 

In summary I suggest the following:

  • Dangerous Bends, Steep Inclines & Roundabouts – Use Grey
  • Approaches to Pedestrian Crossings – Use Buff
  • Approaches to Roundabouts & Junctions – Use Grey unless there is a compelling reason to make the motorist aware of the safety surface and/or a significant hazard ahead.