50 km/h is pretty common descending a hill on a good quality road bicycle. The major difference there is that you will have a properly balanced wheelset (a 700C wheelset alone on a decent roadbike, sans tires, are worth at least $275-350), with gyroscopic stabilizing forces. And you will have brakes. It's also possible on a road bike to aerobrake yourself on a descent by sitting more upright and adding drag.
