Coefficient of traction or friction refers to the maximum frictional force that can be produced between surfaces without slipping. The table below from Noon (1994, see reference below) is an selection of values that represent reasonable averages.
|Surface Type||Coefficient of Friction (µ)|
|Gravel and dirt road||0.35|
|Wet, grassy field||0.20|
|Dry asphaltic concrete||0.65|
|Wet asphaltic concrete||0.50|
|Loose moist dirt that allows tyre to sink about 5cm||0.60-0.65|
Bear in mind that the more water present on the road, the lower the coefficient of friction. As little as 0.4mm of water should be considered “wet” (Tulloch, Stocker 2011).
Noon, R.K. (1994): Engineering Analysis of Vehicular Accidents, CRC Press, Boca Raton.
Tulloch, D. and Stocker, D. (2011) Coal Mine Road Network Surface Friction Report (http://mines.industry.qld.gov.au/assets/mines-safety-health/Coal-Mine-Road-Network-Surface-Friction-Report-2011.pdf)