The magnitude of the impact factor depends on several factors. Which of the following affects the condition of the road surface?

• A. Temperature
• B. Humidity
• C. Wind Speed
• D. Surface Roughness

Answer: (D)

Surface roughness is the direct measure of how uneven the pavement surface is. When a vehicle travels, these irregularities cause the wheel to encounter abrupt height changes, leading to higher vertical accelerations and transient loads. This dynamic amplification is what the impact factor captures, so rougher surfaces increase the impact factor and smoother surfaces reduce it.

Temperature, humidity, and wind speed influence material properties or external loads in other ways, but they do not describe the immediate condition of the road surface itself. Temperature can alter stiffness over time, humidity affects moisture content, and wind affects aerodynamic loads, not the local surface irregularities that drive the impact factor.