5.1 This is the practice for calculating the IFI of the pavement. The IFI has proven useful for harmonization of the friction measuring equipment. F60 and S_p have proven to be able to predict the speed dependence of wet pavement–related measurements of the various types of friction-measuring equipment.2 The two IFI parameters (F60 and S_p ) have been found to be reliable predictors of the dependence of wet pavement friction on tire slip and vehicle speed.

5.2 The IFI parameters, F60 and S_p, can be used to calculate the calibrated friction at another–slip speed using a transformation equation.

5.3 The IFI model given below describes the relationship between the values of wet pavement friction FRS measured at a slip speed of S and between the friction values measured by different types of equipment.

5.4 A significance of the IFI Model is that the measurement of friction with a device does not have to be at one of the speeds run in the experiment. FRS can be measured at some S and is always adjusted to FR60. Thus, if a device can not maintain its normal operating speed and must run at some speed higher or lower because of traffic, the model still works well. In that case S is determined by the vehicle speed (V) which can be converted to S by multiplying V by the percent slip for fixed slip equipment or by multiplying V by the sine of the slip angle for side force equipment.

5.5 This practice does not address the problems associated with obtaining a measured friction or measured macrotexture.