A method is provided for determining a state of a road condition using a linear model-based estimation technique. Two vehicle reference models are defined to represent vehicles operating under non-slippery and slippery road surfaces respectively. An index that reflects the vehicle understeer characteristics is also defined. Indices are determined from the reference models under the non-slippery road surface, the slippery road surface, and from vehicle sensor measurement, respectively. A first root mean square deviation is calculated between the index of reference model under non-slippery road surface and the index calculated based on sensor measurement. A second root mean square deviation is calculated between the index of reference model under slippery road surface and the index calculated based on sensor measurement. A probability analysis is applied as a function of probability density functions for identifying the condition of the road surface between a non-slippery road surface and a slippery road surface.
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