High speeds increase stopping distances greatly. By slowing down a little, you can gain a lot in reduced braking distance. The Effect of Vehicle Weight on Stopping Distance. The heavier the vehicle, the more work the brakes must do to stop it and the more heat they absorb. But the brakes, tires, springs, and shock absorbers on heavy vehicles are designed to work best when the vehicle is fully loaded.
Empty trucks require greater stopping distances, because an empty vehicle has less traction.
It can bounce and lock up its wheels, giving much poorer braking. You can't steer or brake a vehicle unless you have traction. Traction is friction between the tires and the road. There are some road conditions that reduce traction and call for lower speeds. It will take longer to stop and it will be harder to turn without skidding when the road is slippery.