With EGPWS becoming the norm in most airliners this is a further and even better safety net than setting an arbitrary height on the radio alt. that might/might not protect you. EGPWS has a terrain clearance floor function that generates alerts based upon current airplane position, nearest runway centre point position, radio altitude, and a database of hard-surfaced runways whose length is greater than 3500 feet. Penetration of the alert envelope activates the GND PROX lights and generates a “TOO LOW TERRAIN” aural.