In a highway, any car can enter and is flexible to switch lanes and the coordination between cars is mostly a responsibility of the drivers. In the railway everything is pre-routed and no surprises are technically possible — dispatchers and planners have more control than the train driver and there is no room for flexibility — you just stay in your rails. What is air like? Can I build my own plane and just join the air traffic without telling anyone?
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Both, and somewhere in between. There are two main sets of rules for flight:
VFR — visual flight rules. Small aircraft can basically take off and fly any route they want, whenever they want, without much planning or preparation. Sometimes they don’t even have to talk to anyone by radio. They navigate visually, looking at landmarks and such. However there are “rules of the road” that VFR flights have to follow, especially near airfields, and usually VFR flight is only allowed at low altitude, away from big commercial airports, when the pilot can see the ground and surrounding airplanes. Big commercial aircraft are not allowed to do this.
IFR — Instrument flight rules. Much more complicated. Pilots generally have to file a flight plan saying where they’re going, how they’re going to get there, and when they want to leave. They’re usually in constant communication with air traffic control to follow their plan. They navigate using radio instruments and GPS. The rules are *much* more strict and complicated, but they allow pilots to fly big commercial aircraft into major airports in very bad weather. Pilots need special equipment, training and a specific type of license to do this.
Basically, to use your analogy, small personal airplanes work a lot like cars, big commercial aircraft work a lot more like trains.
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