the kinetic energy in the faster case is 4x as much. the brrakes must do an amount of work that is equal to the original kinetic energy. the brakes exert the same force in either case, so the distance will be 4x as long for the faster case.
If it is the same vehicle, then the 60mph vehicle has more kinetic energy since it is moving faster. Therefore, it requires more energy to stop, and if it is the same car with the same beak system, the braking distance of the 30mph car will be significantly shorter than the 60mph car.