#29 Road Surface Friction

Road surface friction is directly related to skid resistance and, therefore, crash risk. Low surface friction can contribute to crashes involving roadway departure, intersections, turns and other situations where vehicles frequently stop or slow significantly, such as pedestrian crossings. While all vehicle types can be affected, motorcycles in particular are at risk due to the small contact area between tyres and road surface. High friction surfaces offer better skid resistance, especially under wet conditions, and are necessary at locations with the following characteristics:

• Horizontal curves and downhill slopes

• Interchange ramps

• Intersection approaches

• Pedestrian crossing approaches

• Locations with rear-end crash history

  
  

High Friction Surface Treatments (HFST) can be applied as targeted treatments at locations with these characteristics.

Source: Dave Jones, Polarenviro

Effectiveness

As reported by PIARC[1], in a general sense, crash rates increase with a decline in road surface skid resistance. According to the US Federal Highway Administration (FHWA), HFST can reduce injury crashes by 63% at ramps and by 48% at horizontal curves. Application of HFST at intersections saw a 20% reduction in total reported intersection crashes.

Calcined-bauxite is a recommended aggregate material due to its ability to maintain skid resistance over time relative to other materials. Crash modification factors (CMF) and benefit-cost (B/C) ratios for this material were examined in a large-scale US study[2]. Based on sites for which sufficient data were available, B/C ratios of 6.00 and 18.7 were estimated for curves and ramps, respectively, indicating clear cost-effectiveness.

Testing and Monitoring

Friction properties and skid resistance deteriorate over time, with the rate of decline depending on a range of factors including the types of aggregates used, binder and base properties, traffic volumes and vehicle types, operating speeds, and the frequency of vehicle accelerations and decelerations, among others. Surface integrity and deficiencies should be monitored accordingly, and ideally proactively, to assist timely identification and treatment. Monitoring can help identify and prioritise the targeting of sites for treatment, while pre- and post-treatment friction testing will indicate the level of improvement.

Visual observation can identify a range of road surface problems but can be limited for accurately detecting friction deficiencies and cannot measure them accurately. Friction measurement devices of various types are therefore also used for static and dynamic testing of both longitudinal and side-force friction. According to the FHWA, “friction data for safety performance is best measured with Continuous Pavement Friction Measurement (CPFM) equipment”.

[1] PIARC Technical Sheet: Road Surface Conditions

[2] Merrit et al. (2020). Developing crash-modification factors for high-friction surface treatments. Washington, FHWA.