Friction tests on contaminated road surfaces

Anecdotal evidence indicates that the presence of contaminants in the tyre/road interface reduces the friction generated between the tyre and the road and that the sudden reduction in friction over the area affected can contribute to a driver losing control of a vehicle. This report presents the results of an investigation of the effect of various types of road surface contamination on the measured friction. Tests were carried out on two surfaces at an ex-USAF base at Bentwaters Park, Suffolk - one asphalt, the other concrete. Peak and sliding coefficients of friction were measured at speeds up to 80km/h with a locked-wheel pavement friction tester (PFT) and two skid cars. The PFT used a completely smooth test tyre and the skid cars were fitted with normal road tyres in roadworthy condition. The contaminants with which these surfaces were tested were diesel oil, engine oil, clay and coarse sand; additional tests were carried out after a proprietary absorbent material had been applied to the diesel oil and engine oil. The main conclusion is that friction coefficients for cars with tyres in good condition are no worse than might be expected for many road surfaces when wet, but that tyre tread is critically important to this performance: friction coefficients of practically zero can be observed with smooth tyres, even with an underlying surface that is highly skid resistant.