## 6 Surface Characteristics Assessment
### Fixed and Braking Test Methods
However, most of these recommendations only contain recommendations for surface condition controls, self-wetting system accuracy and general requirements, without specific procedures, especially when interpretating the outlying results. International standards have a similar issue [68]; hence, it should be recommended to incorporate the above-mentioned recommendations. In the 1990s, the World Road Association, known as PIARC, performed an assessment of different friction testing techniques and developed the International Friction Index (IFI) [104]. The study, finalized in 1995, consisted of an investigation of friction and pavement texture testing methods. During the investigation, a correlation between 74 devices representing sixteen countries was found, and the Golden Value Friction Number (GF 60) and Golden Value Speed Number (GS) were evaluated. The correlation between each device and the golden number of that parameter was reported, and methods of correlation investigated for other devices were presented [104]. In a similar study, the International Friction Index for various devices was compared to the PIARC results and it was concluded that the correlation coefficients obtained by PIARC did not match their experiments [105]. The method of calculation of the predicted Golden Speed Value (S\({}_{\text{P}}\)) also does not correlate with the experimental data. Furthermore, it was also concluded by others that the PIARC method did not correlate with new and independent experimental data [106]. Despite these shortcomings, the data obtained by PIARC can be used for friction assessment and comparison of different devices [107]. However, the factors that affect the results must be considered when comparing the PIARC results to other CFME trial results. Influence of Weather Conditions on Friction Measurements  
Weather conditions, such as temperature and humidity, can significantly affect runway friction [108]. Consequently, measured friction values differ from day to night, as well as from season to season, and climate to climate. This was confirmed by long-term analysis of CFME results performed in Australia [97], as shown in (Figure 19). The summer (December) values were generally 7% lower than the winter (June) values. Researchers from New Zealand performed a similar analysis of data collected on highways, to develop a correction equation for the weather conditions [70]. The result was a model for estimating the yearly seasonal fluctuations in skid resistance, specific to the climate in New Zealand. \[BPN=BPN_{terminal}-5 cos(2\pi/365.25\times JDay) \tag{5}\]  
\[GN=GN_{terminal}+0.002 cos(2\pi/365.25\times JDay) \tag{6}\]  
where \(BPN\) and \(GN\) are British Pendulum Number and Grip Number and \(JDay\) is a Julian calendar year. Equation (6), however, according to the authors, needs to be treated with caution as it is derived from data covering only one climatic region and aggregate type. The influence of temperature on friction measurements for the British Pendulum and Grip Tester is shown in Figures 20 and 21. Reported temperature sensitivity for the Grip Tester is similar to temperature sensitivity obtained by other researchers [109]. Similar results were obtained in another study, where both surface and water temperature influences on a friction coefficient measured with a Dynamic Friction Tester were analyzed [110]. According to the results, temperature can affect the friction coefficient significantly (Figure 22). Figure 19: The results of friction measurements on the Calibration Strip at an Australian Airport during the year [97]. Figure 20: Correlation between British Pendulum Number and temperature as a function of road surface type [70]. In all of the presented studies, high temperature reduced the measured friction value. One of the possible reasons for this effect is an increase in the wetting ability of the liquid with increased temperature. That assumption, however, needs to be studied further.