Marine and air navigation can still learn from each other. Mariners are following aviation in adopting some routing and control but aviation could follow mariners in their refusal to accept routing and control unless it is cost effective. However it may be easier in the marine case to measure the cost and probability of a collision than it is in the aerial case. But many states have established mandatory airways and routes; originally their navigational aids were very useful and in the event of forced landing the search and rescue people knew where to look. But many airliners now have self-contained navaids like the inertial navigation system (INS) or doppler so that they can navigate off airways, and also forced landings are rare. In the history of air traffic control at first it applied only around airports; when it spread to routes many aircraft were quite happy to fly off airways. But now airways have proliferated and have become customary and respectable to airline pilots so that few complain when some states force aircraft to follow lengthy and costly routes. The only virtue of airways is their freedom, theoretically, from uncontrolled traffic and from military dangers. If A.T.C. were confined to those situations where the risk of collision without A.T.C. would be worse than, say 1 in 10,000,000, flying would be a little more expeditious, but everyone feels secure within the system and insecure, guilty and uncomfortable outside it, so A.T.C. is used universally by airliners.