## 1 Introduction
Compared to sliding measurements like those performed with the BPT and the DFT, field testing is a more precise technique to capture the real frictional properties of pavements. Field testing has the advantage of being able to capture ongoing, in situ data, which can result in a more comprehensive assessment of pavement friction. A thorough set of experiments was carried out by the Permanent International Association of Road Congresses (PIARC) with the goal of contrasting different techniques for measuring texture and friction. The International Friction Index (IFI) is a standardized and harmonized index that was developed by researchers based on a set of empirical relationships they established from the data gathered during the 1992 trials [16]. This model, which is now widely known as the PIARC model, combined side-force, fixed-slip, and locked-wheel friction techniques with macrotexture measurements. By adding these elements, the model was able to take into consideration the well-established effects of surface texture and speed on pavement friction. Several investigations have examined the fundamental ideas behind the IFI, paying special attention to how the speed constant (Sp) functions in IFI computations. Although the Sp parameter is typically associated with the macrotexture properties of pavement surfaces in the literature currently in publication, some research indicates that when the relationship is examined in a device-specific context, stronger correlations between Sp and Mean Profile Depth (MPD) appear. A common limitation of the standard ASTM-based IFI methodology is the slip speed dependency that is frequently seen in device calibration. In some cases, it has been demonstrated that altering the Sp parameter can significantly reduce this dependency [17; 18]. Moreover, the IFI model assumptions do not hold for treatments like HFST, which often maintain constant friction across slip speeds. This limits the usefulness of the speed parameter (Sp), particularly on surfaces with high macrotexture. The IFI model, as defined in ASTM E1960-07, is also validated within a narrow range of MPD and DFT values, making it less reliable for HFST applications [19; 20; 21; 22]. There is a need to optimize or reformulate IFI using updated models that can better capture nonlinear relationships between texture and friction and also for the higher range of MPD. Numerous studies have investigated various aspects of HFST, including materials and their properties, friction performance of different aggregates, safety evaluations, benefit-cost (BC) ratios, and issues associated with HFST implementation [23; 24]. Despite these efforts, some critical problems remain, particularly the need to understand the skid resistance of this surface treatment. The main objectives of this study are to:  
1. Develop and evaluate alternative models for predicting IFI from laboratory measurements of COF and MPD on HFST surfaces to address the limitation of the standard IFI methodology. 2. Identify the most accurate model through performance metrics. 3. Assess the sensitivity of IFI predictions to COF and MPD. These objectives provide a useful laboratory assessment of HFST friction resistance that is made possible by the results, which are essential for well-informed material selection and design optimization.