What can make total stopping distance difficult to estimate?

Total stopping distance combines several factors that can vary greatly, making it challenging to predict accurately. This distance is influenced by a combination of the driver's reaction time, the vehicle's speed, the road conditions, weather, and the vehicle's braking capabilities. For instance, on wet or icy roads, the stopping distance increases significantly compared to dry conditions. Additionally, different drivers may have different reaction times based on their experience, alertness, and other situational factors.

Thus, the variability of these elements contributes to the difficulty in estimating a precise total stopping distance in any given situation. The other options do not reflect this reality; for instance, it is not easy to calculate due to the numerous influencing factors, it certainly varies depending on conditions rather than being the same in all scenarios, and it differs for various vehicles based on their size, weight, and braking systems.