It IS true, in general, outside of Restricted [Military] airspace. The sole exception is when the airplane cannot be safely flown slower, e.g., a heavy airplane after takeoff that has a minimum clean speed >250. That exception is available without specific clearance EXCEPT in a few airport areas such as ORD, where the speed limit is specifically cited.
A controller cannot approve faster. A waiver must be gotten from the FAA for airshows and other specific events.
Last edited by Intruder; 21st Dec 2016 at 20:36.