## 3 Methods for the Characterisation and Evaluation of the Skid Resistance of Asphalt Pavements
### Traditional Stid Resistance Testing Methods and Indicators for Asphalt Pavements  
Traditional test indexes for pavement skid resistance are as follows: friction coefficient of pavement, dynamic friction coefficient, transverse friction coefficient, longitudinal friction coefficient, and slip index. The coefficient of friction of a road surface can be quantified by a British pendulum tester, which utilises the free-fall motion of a pendulum at a fixed height to convert the potential energy of the pendulum into kinetic energy. This is achieved through the use of a rubber pad on the pendulum which comes into contact with the road surface at  
Figure 10: Scanning electron microscope [74]. a slip speed of 10 km/h [75], thereby determining the characterised friction coefficient of the road surface under wet conditions. The friction coefficient of pavement can be employed to assess the skid resistance characteristics of a road surface when a vehicle is in motion. BPN is associated with the aggregate type, shape, and particle size. The method is inexpensive and requires minimal equipment and operation, making it a popular choice for laboratory and engineering tests. Nevertheless, the test outcomes are susceptible to variability resulting from operator influence, the thickness of the water film, and the ageing condition of the rubber block [76]. A British pendulum tester is depicted in Figure 11.  
Dynamic friction testers (DFTs) are used to determine the dynamic coefficient of friction between two sliding contact surfaces. In the Field Test Methods of Highway Subgrade and Pavement (JTG 3450-2019) [77] implemented by the Ministry of Communications of China on 1 September 2008, DFT was identified as one of the test methods for determining the coefficient of friction of pavement. The coefficient of friction, as measured by the DFT at a speed of 60 km/h, is employed as the standard measurement, designated as DFT\({}_{60}\). Nevertheless, the test results may be inaccurate in the presence of pollutants on the road. The test setup is depicted in Figure 12.  
Figure 11: British pendulum tester. Figure 12: Dynamic friction tester. Pavement skid resistance can be further subdivided into longitudinal and transverse skid resistance and is characterised by the transverse and longitudinal friction coefficients, respectively. The transverse force coefficient is the ratio of the transverse force to the vertical force. The transverse force coefficient SFC60 was quantified by the lateral force coefficient test vehicle at a velocity of 60 km/h while the road surface was kept in a wet state, as illustrated in Figure 13. However, due to the high cost of the test equipment and its inability to test small areas and other specialised pavements [78], the transverse force coefficient test is somewhat limited in its use and dissemination. The longitudinal friction coefficient determines the emergency braking distance of a vehicle and is closely linked to driving safety. [PERSON] et al. [79] used the pavement longitudinal coefficient of the friction testing vehicle. Since the longitudinal friction coefficient test vehicle must be on a road section of specific length (generally \(\geq\) 100 m) to obtain data, the test range needed to be determined in advance. If the test section was only laid out temporarily at the site to carry out the test, the test cost would be too high. The device is shown in Figure 14.  
Figure 14: Road surface longitudinal friction coefficient testing vehicle. Figure 13: Road surface transverse force coefficient testing vehicle. The slip resistance of the road surface can also be expressed using the slip index (SN). The test equipment is a locked wheel tester with the test wheel which can be fully locked. The tyres were inflated to a pressure of 165 kPa and were of the standard tread type. In the test project, the water film thickness was 0.5 mm, which equates to the distance between the test tyre and the pavement being 0.5 mm.