Guidance

Section 4: measuring skid resistance

Published 17 December 2024

Applies to England

SCRIM

Skid resistance is a measure of the road surface contribution to the frictional forces developed between a vehicle’s tyres and the road when accelerating, braking or cornering. It is generally measured after the road has been wetted and data can be collected in various ways. The method most widely used on roads is the Sideway-force Coefficient Routine Investigation Machine (SCRIM, Figure 1). This is a water tanker with a test wheel mounted at 20 degrees to the direction of travel. The test wheel is free to rotate and is subject to a 200-kilogram vertical load. Water is sprayed onto the road surface immediately in front of the test wheel, which is permanently in a controlled-skid condition, and the frictional force developed perpendicular to the plane of the test wheel is measured.

Figure 1: Example of SCRIM machine

On trunk roads, the whole length of Lane 1 is surveyed each year, including slip roads, this being the lane that carries the majority of heavy vehicles. Roundabouts are also surveyed. Local authority practices vary somewhat, with authorities basing their strategy for managing skid resistance on the traffic flow and accident characteristics of their network as part of their Highways Asset Management Plan (Department for Transport, 2005).

Skid resistance data are dependent on the vehicle speed. On motorways and dual carriageway all-purpose trunk roads where the posted speed limit is greater than 50mph, SCRIM surveys are carried out at a vehicle speed of 80km/h (roughly 50mph). On all other roads, SCRIM surveys are carried out at 50km/h (just over 30mph). However, it is often not practical to maintain a constant speed in a network survey, so a facility is provided to correct skidding resistance data to give equivalent values at 50km/h. 

The skidding resistance of road surfaces has been shown to vary during the year. In the winter, the action of the weather and gritting results in an increase in skidding resistance. Trafficking and drier weather during the summer serves to polish the surface and reduce the measured skid resistance. Therefore, the measurements obtained can be dependent on the time of year that the survey was carried out. To reduce the effect of seasonal variation, the testing season is restricted to 1 May to 30 September.

As a result of this variation in the skidding properties, until 2005, SCRIM surveys of the trunk road network were carried out on one third of the network each year, with each length surveyed 3 times in the year (at the start, middle and end of the SCRIM survey season).

However, there was a change in approach to the surveying of trunk roads from 2005, such that skidding resistance measurements are now undertaken each year under a single annual survey regime. The surveys are scheduled such that, over a 3-year period, the network is surveyed early, middle and late in the test season in successive years, for example, a length that was surveyed early in the first year will be surveyed in the middle of the season in the second year and then late in the season in the third year.

National Highways (NH, formerly Highways England) has implemented an annual correlation trial for SCRIM survey vehicles, which are currently carried out by TRL. All SCRIM vehicles undertaking surveys on trunk roads are required to pass the trial in order to undertake surveys on the trunk road network. Other SCRIM survey operators are also able to attend the trials, although it is not compulsory. However, local authorities commissioning SCRIM surveys typically expect that the SCRIM vehicles used on their network will have passed the trials, and therefore in practice it has been found that all SCRIM vehicles operating in the UK attend the trials. In the trials, the SCRIM vehicles are required to undertake surveys on a number of sites with different levels of skid resistance, and the data is then compared to identify outliers. The trials therefore aim to ensure consistency across the fleet of vehicles operating in the UK.

Reporting SCRIM skid resistance data

SCRIM skidding resistance data are recorded continuously and stored as an average. Typically, the SCRIM values are reported as SCRIM Coefficients over 10m lengths.

The SCRIM Coefficients are processed to correct for temperature and speed (to obtain corrected SCRIM Coefficients) and then these are further processed to obtain Characteristic SCRIM Coefficient values (CSC), in which some allowance has been made for seasonal effects on the skid resistance.

Where the survey procedure has been to carry out surveys of one third of the network each year, with each length surveyed 3 times in the year, the characteristic value is calculated as the average value of the corrected SCRIM Coefficients over the 3 surveys. This is defined as the Mean Summer SCRIM coefficient.

Under the single annual survey regime, the network surveyed is broken up into regions, or localities, that have similar climatic conditions. The average corrected SCRIM Coefficient in the preceding 3 years is calculated for each locality, along with the average corrected SCRIM Coefficient for the current year. The ratio of these values defines a factor that is used to scale each of the corrected SCRIM Coefficients obtained in the particular locality in the current year.

Typically, the 10m CSC data are averaged into longer, 100m lengths, for further analysis.

The calculation and application of a coefficient that is based on skidding resistance data from the previous 3 years means that the skidding resistance value provided by National Highways for a given year is influenced by the values from other recent years. These can cause the measured value for the most recent year to be adjusted upwards or downwards. This process is designed to smooth out fluctuations in the measured values to give a more reliable long-term trend.

Using SCRIM skid resistance data

The degree of skidding resistance required at a given site on the road network depends on the site category (for example, junction approach, bend). An investigatory level is established by the highway authority for each site category. If the skidding resistance at a site is found to be at or below the investigatory level, the highway authority policy is normally to carry out a site investigation to determine whether the existing level of skidding resistance is likely to cause an accident problem. Note that, if a road is below the investigatory level, it does not necessarily mean that the road is unsafe, it indicates a need for further investigation to determine the need for maintenance.

For trunk roads, the investigatory levels for each site category are specified in the Design Manual for Roads and Bridges, HD28/04. Local highway authorities may either apply these levels or develop a local policy. Local policies will often be based on HD28/04 but adapted according to local practice.

As noted in technical note section 2, SCRIM data is used when reporting the surface condition of trunk roads in the report. The data used in the report (from 2004) is obtained by determining whether any sub-section has a CSC which falls 0.05 or more below the Investigatory Level. If so, then this length will be counted within the total length reported to be in need of maintenance.

GripTester

The GripTester offers an alternative to the use of SCRIM for the measurement of friction on certain roads and is included here for information. The GripTester is a 3-wheeled trailer. The third wheel being braked to produce a drag (skid) and both drag and load are continuously monitored. The friction coefficient (such as load and drag) is calculated and transmitted to a data collection computer held in the cab of the towing vehicle.

Due to its smaller size, the GripTester has a wider range of application, including horizontal signs, footways and pedestrian precincts as well as highway surfacing (BS 7941 Part 2). Because of the different measurement method of the device, it is not possible to directly compare GripTester results with the data provided by SCRIM. However, research work is being undertaken to determine whether robust conversion factors could be developed.

There is currently no formal process of accreditation or correlation for GripTester devices.

Further information on the SCRIM

The above paragraphs provide summary information on SCRIM and GripTester, and the data they provide. Further information can be obtained in the following references:

Design Manual for Roads and Bridges (DMRB) – CS228 “Skidding Resistance”, (National Highways).

BS7941 Part 1, “Methods for measuring the skid resistance of pavement surfaces. Sideways Force Coefficient Routine Investigation Machine”, BSI.

BS7941 Part 2, “Surface friction of pavements – test method for measurement of the surface skid resistance using the GripTester braked wheel fixed slip device”, BSI.

Department for Transport (2005). “Well maintained highways. Code of practice for highway maintenance management”. The Stationery Office, London.

Impact of COVID-19

In 2020, the early part of the SCRIM survey season coincided with the first national lockdown for COVID-19, during which there were dry weather conditions and lower levels of traffic. It has been observed that some SCRIM surveys undertaken in this early period had lower skid resistance values. The effect of this may be observed in an authority’s data, with a significance that will depend on the method used for seasonal correction of the SCRIM data.

Contact details

Road condition statistics

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