A method for providing, for a portion of a road , an approximation of a roughness figure in accordance with a roughness index, the method comprising: - receiving speed data of a first vehicle travelling along the portion of the road and receiving , from a measuring device carried on the first vehicle, measured acceleration data of the device perpendicular to the road surface, the first vehicle falling in a first class of a plurality of classes of vehicles, a respective set of speed-based conversion equations being pre-derived for each of the plurality of classes of vehicles, a first set of the respective sets of speed-based conversion equations (46.1) relating to the first class of vehicles, the first set of speed-based conversion equations comprising a plurality of conversion equations, wherein each of the conversion equations of the first set relates to a different predetermined speed; - processing the acceleration data, to provide a parameter value (x1) relating to the acceleration data for the portion of the road; - characterized in that based on the speed data a first speed-based conversion equation is selected from the first set of speed-based conversion equations, in that the first speed-based conversion equation is used to convert said parameter value (x1) into an approximation (y1) of a roughness figure for the portion of the road in accordance with the roughness index and in that the parameter value is a coefficient of variation (CoV) for the portion in the form of a ratio of a standard deviation, σ, and a mean, μ, of the acceleration data, that is CoV = σ z − Acceleration μ z − Acceleration × 100 , thereby allowing acceleration data from vehicles of different classes and travelling at different speeds to be compared. - receiving speed data of a first vehicle travelling along the portion of the road and receiving , from a measuring device carried on the first vehicle, measured acceleration data of the device perpendicular to the road surface, the first vehicle falling in a first class of a plurality of classes of vehicles, a respective set of speed-based conversion equations being pre-derived for each of the plurality of classes of vehicles, a first set of the respective sets of speed-based conversion equations (46.1) relating to the first class of vehicles, the first set of speed-based conversion equations comprising a plurality of conversion equations, wherein each of the conversion equations of the first set relates to a different predetermined speed; - processing the acceleration data, to provide a parameter value (x1) relating to the acceleration data for the portion of the road; - characterized in that based on the speed data a first speed-based conversion equation is selected from the first set of speed-based conversion equations, in that the first speed-based conversion equation is used to convert said parameter value (x1) into an approximation (y1) of a roughness figure for the portion of the road in accordance with the roughness index and in that the parameter value is a coefficient of variation (CoV) for the portion in the form of a ratio of a standard deviation, σ, and a mean, μ, of the acceleration data, that is CoV = σ z − Acceleration μ z − Acceleration × 100 , thereby allowing acceleration data from vehicles of different classes and travelling at different speeds to be compared. <EOS>