It depends on which type of cruise control it is. Normal cruise control uses only the accelerator to keep the speed up to the selected speed so, going downhill the chosen speed can be exceeded. Adaptive cruise control uses accelerator and brakes to keep to the chosen speed so, within its limits of accuracy, will maintain the chosen speed up hill and down dale. It will also slow you down if you begin to get too close to a vehicle in front of you travelling slower than you are.