these words, written more than 40 years ago, seem even more prophetic today:
Hank Searls says in his prologue(the crowded sky): "The airway is not a highway, but an invisible alley, sometimes of infinite height. Day or night, its traveler depends for safety not as the motorist does on keeping right or left of a center line, but on holding a promised speed and an assigned altitude. If he speeds up, he will overtake his predecessor; if he climbs or descends inad-vertently, he is suddenly in an opposite stream of traffic approaching with the speed of sound."