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.

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

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.

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.

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

1.1 This practice covers the calculation of the International Friction Index (IFI) from a measurement of pavement macrotexture and wet pavement friction. The IFI was developed in the PIARC International Experiment to Compare and Harmonize Texture and Skid Resistance Measurements. The index allows for the harmonizing of friction measurements with different equipment to a common calibrated index. This practice provides for harmonization of friction reporting for devices that use a smooth tread test tire.

1.2 The IFI consists of two parameters that report the calibrated wet friction at 60 km/h (F60) and the speed constant of wet pavement friction (S_p).

1.3 The mean profile depth (MPD) has been shown to be useful in predicting the speed constant (gradient) of wet pavement friction.

1.4 A linear transformation of the estimated friction at 60 km/h provides the calibrated F60 value. The estimated friction at 60 km/h is obtained by using the speed constant to calculate the estimated friction at 60 km/h from a measurement made at any speed.

1.5 The values stated in SI (metric) units are to be regarded as standard. The inch-pound equivalents are rationalized, rather than exact mathematical conversions.

1.6 This standard does not purport to address all of the safety concerns, if any associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.