Measuring skid resistance
The skid resistance of a road surface enables vehicles to manoeuvre safely and it is therefore necessary to regularly monitor the skid resistance of the road network. These measurements are made using various test devices that commonly use a rubber wheel or slider that is made to slip against the road surface, in the presence of water, whilst monitoring the force generated.
A fleet of Sideway-force Coefficient Routine Investigation Machines (SCRIM)® - developed by and still made under licence from TRL - is used to continually measure the skid resistance of the trunk road network.
A freely rotating wheel, on the vehicle's near side, fitted with a smooth rubber tyre and angled at 20° to the direction of travel of the vehicle, is applied to the road surface under a known vertical load. A controlled flow of water wets the road surface immediately in front of the test wheel so that, when the vehicle moves forward, the test wheel slides in the forward direction along the surface. The force generated by the resistance to sliding can then be related to the wet skid resistance of the road surface.
Skid resistance standards, as published in the Design Manual for Roads and Bridges are based on measurements taken with SCRIM. Each part of the network is assigned a site category based on its characteristics and the potential for skidding accidents. Each site category has an associated investigatory level (IL). Where measurements with SCRIM fall below the IL, a site investigation is triggered to determine the need to treat the site to restore skid resistance.
TRL operates a SCRIM, which is used:
- To provide data to support the development of skid resistance standards;
- To act as a test bed for evaluating technical developments to SCRIM, before rolling them out to the rest of the fleet;
- As a reference device during annual correlation trials of the UK SCRIM fleet;
- To support research into the skid resistance behaviour of surfacing materials
GripTester® (GT) is a small trailer for measuring wet skid resistance. Designed and developed by Findlay Irvine Ltd, initially for use on airfields and helipads, it is now used by many local authorities on their road networks. TRL operates one GT on behalf of Highways Agency.
The GT uses the fixed slip principle: it has two ‘drive wheels' linked by a gear and chain system to the test wheel such that it is braked to slip at a fixed rate. Water is fed from a container carried in the towing vehicle (typically a transit van) to wet the road just in front of the test wheel, which is fitted with a standard smooth tyre. The horizontal and vertical forces acting on the wheel are measured via an instrumented axle, and used to calculate a "GripNumber" to represent the skid resistance of the road.
Local authorities use the GT primarily because of its small size, added manoeuvrability, and low cost. It can be used to survey on road sections that are inaccessible to SCRIM, and it can test in any path across the width of the towing vehicle.
Pavement Friction Tester
The Pavement Friction Tester (PFT) operated by TRL Ltd was acquired in 1996 by the Highways Agency. The device is manufactured by Dynatest (formerly K J Law).
The PFT uses the locked-wheel method to measure tyre/road friction; the test wheel is braked so that it locks and skids along the surface before being released. Vertical and horizontal forces on the test wheel are recorded every 1/100th of a second. Values for the peak and sliding friction at a known speed are then calculated and a PFT measurement is referred to as a Friction number (Fn).
Water is normally pumped from the tank in the tow vehicle (a Chevrolet Silverado 3500 in this case) to wet the road in front of the test wheel at a controlled rate. The water feed can be turned off to make dry measurements for special purposes. Purpose-made standardised tyres are usually used for PFT tests, but many standard car tyres can be fitted if desired.
Unlike SCRIM and GripTester, the PFT is not suitable for continuous network surveys. However, the locked-wheel technique is the best way of measuring sliding friction directly at a specific speed.
Uses for the PFT include:
- Investigating the frictional performance of existing roads at different speeds;
- Provision of detailed information for accident/incident investigations;
- Helping in the development of new road surfacing materials;
- Assisting in the development of tyres and vehicle braking systems.
British Pendulum Tester
Originally developed by TRL in the 1960s to study the problems in design and maintenance of public highways, the pendulum tester is used to test the skid resistance of existing road surfaces. It is also used in conjunction with a large scale polishing machine to determine the polishing resistance - polished stone value (PSV) - of aggregate before it is used in road surfaces.
The Tester is based on the pendulum principle. A pendulum consisting of a tubular arm rotates about a spindle attached to a vertical pillar. At the end of the tubular arm a head of constant mass is fitted with a rubber slider. The pendulum is released from a horizontal position so that it strikes the sample surface with a constant velocity. The distance travelled by the head after striking the sample is determined by the friction of the sample surface. A reading of Skid Resistance Values is obtained.