## 3 Methodology
### Highway Rockfall Susceptibility
The terrain of Guizhou Province is high in the west and low in the east, and the average elevation reaches more than 1000 m. At different elevation ranges, there is significant variability in vegetation cover, rainfall, temperature variations, and the intensity of human engineering activities. These factors directly or indirectly affect the occurrence of geologic hazards. 2. Slope. Slope reflects the steepness of the surface of the rockfall hazard site, and is generally expressed as the ratio of the vertical distance from a point on the slope surface to the foot of the slope and the horizontal distance from the foot of the slope. Some slopes are potentially hazardous. The steeper the slope, the more water and soluble salts will gradually accumulate down the slope, directly affecting the vegetation distribution on the slope [39]. The movement of water within the slope, the denudation and material distribution of the rock and soil bodies on the slope surface, and the stress characteristics within the slope body will be different for different slope locations, and the probability of a rockfall will vary. The probability of rockfalls will also change accordingly. The greater the slope, the more likely the rocks are to be eroded, leading to the occurrence of rockfall accidents. Moreover, the greater the slope, the greater the kinetic energy of the rockfall. 3. NDVI. The NDVI is calculated from landsat8 OLI remote sensing satellite bands. The NDVI can separate vegetation from water and soil, can objectively reflect changes in the amount of vegetation cover, and is the best indicator of vegetation growth status and vegetation cover [49]. The vegetation significantly influences the stability of a slope. Studies have shown that vegetation can strengthen the shear capacity of soil through its root system, which has the effect of soil consolidation and slope protection. As the vegetation cover increases, the stability of the slope improves, and the probability of rockfalls decreases. 4. Aspect. Aspect is expressed as the angle between the projection of the normal direction of the slope on the plane and the due north direction. Aspect has an effect on both the number of days of sunshine and the total radiation received by the slope [44]. Due to differences in the amount of solar radiation received, the evaporation of water from the slope surface, the distribution of vegetation on the slope surface, the pore water pressure, and the wet-dry cycle of the slope will vary from one slope direction to another, which in turn will lead to rainfall, vegetation cover, erosion, and weathering of the slopes. 5. Distance from fault. A fault is a structure in which the earth's crust is fractured by force, and significant relative displacement of rock masses occurs along both sides of the fracture plane [13]. Faults disrupt the rock and soil structure in the vicinity, and the integrity of the slope is compromised. The closer the fault, the more discontinuous the rock, and the looser the soil, the higher the probability of rockfalls. 6. Relief amplitude. Relief amplitude is the difference between the elevation of the highest point and the elevation of the lowest point in a given area. It is a macroscopic indicator that characterizes the topography of a region [58]. Relief amplitude is mainly the result of tectonic movements and surface erosion and represents the degree of regional surface denudation and cutting. As relief amplitude increases, the probability of rockfalls increases. 7. Lithology. Lithology refers to some properties that reflect the characteristics of the rock. In different lithologies, shear strength is distinct; permeability is also different; the stability of the slope, destabilization degree, and resistance to stability are different; and the ease of occurrence of geologic disasters is also different [41]. Lithology directly affects the size and extent of rockfalls.