There is a small hill as you are heading south, then there is a bigger valley on the other side, the speed limit is 40. Your vehicle is using more power to get up the hill. Then once your car peeks the hill the extra power is converted into speed on the downward side which is steeper and longer. This make you vehicle go to about 55 or more depending on the vehicle. At the bottom in a driveway or a side street is where the police car is waiting with the speed gun to catch you. You are caught going you fastest for 1 to 2 seconds as the converted energy is lost. If you keep the power on after cresting the hill you speed will be even higher.