Sometimes we are afraid that the bucket will fall off in the middle of the road, but in reality, it never falls off.
For example, suppose a bucket of water is swirled around by hand. If you spin it fast enough, the water will not spill even if it is upside down, but if you spin it too slowly, the water will spill.
The same principle applies to the roller coaster and the bucket of water. If the water is moving in a circle, the faster it spins, the more force it exerts to move away from the center, so it does not fall.
Of course, if the speed is too slow, the roller coaster will fall down, just like a bucket of water. This would cause a major accident and be too dangerous. That is why roller coasters are equipped with safety devices that connect the rails to the roller coaster under the wheels, so that the roller coaster will never fall off in case of an accident.