Final answer:
You must slow down on gravel or dirt roads due to less traction, which can lead to skidding or losing control if driving too fast.
Step-by-step explanation:
You have to slow down when driving on gravel or dirt roads because these surfaces have differing frictional properties compared to paved roads. Gravel and dirt provide less traction, which means your vehicle's tires cannot grip the road as well, increasing the likelihood of skidding or losing control, especially at higher speeds. Therefore, to maintain safety and control of the vehicle, it's necessary to reduce speed on these less stable surfaces.