I have heard of the "highways in the sky." How do Pilots navigate?
As I detail in my bookThis is Your Captain Speaking, in the old days we needed ground-based navaids (navigation aids) to show us how to get from A to B. Like the interstate system on the ground, a system of routes was devised in the sky. The lower routes were Victor routes (as in V147) while the highest routes were created after jets came of age --Jet Routes (hence J65). Radio beacons on the ground told the pilots where they were in relation to the beacon. Pilots would fly from one beacon to the next on "radials." Each navaid radiated beams for each of the 360 degrees on the compass, and a Pilot could fly each beam fairly precisely. That guidance error increased as you got away from the station. At 60 miles out, it was not unusual to be plus or minus a mile or more off where you thought you were. Still, that was fine for the traffic back then.
Then came satellites. The Global Positioning System (GPS) that has become an integral part of cars, phones and hiking aids was created by the Department of Defense to aid in getting weapons on target. A byproduct of that meant you could navigate with extreme precision --and you didn't need any ground-based naviads to do it. Launching satellites is hugely expensive but not too bad when compared to maintaining thousands of ground-based navaids that quit, get struck by lightning, and almost always are out in the sticks. GPS is way better. Especially when you are over the ocean. No naviads out there! Just create points in space, and the GPS can connect the dots and fly it with extreme precision. It is so exact that, occasionally, we end up flying through each other's wake (aircraft leave waves of disturbed air behind them that are similar to the waves caused by boats on water) when there is no wind blowing to break it up. Alas, progress has its drawbacks....
OK, so that is how we navigate. The why is a little more interesting.
Big airports get a bunch of traffic. Los Angeles (LAX) gets its share, but it's kind of out by itself. The Air Traffic Control (ATC) folks figure out a way to get planes into and out of LAX in each of the directions planes want to go. LAX accepts traffic from the northeast flying from the east coast. Vegas traffic has to be routed into the flow from the northeast too. Phoenix traffic comes into LAX from due east. San Francisco Bay traffic has to be worked in and out to the north. Huge amounts of traffic come from the west across the Pacific. That gives you an idea of what an airport needs in terms of "roads" in and out of town.
Now, imagine six LAX's close to each other!
JFK is only a few miles from Newark and Laguardia. Just down the coast is Philly, Baltimore, and D.C. A map of the northeast route structure is a busy place. Then throw in special-use airspace (military training areas) and security areas like over D.C., and things get really complicated. Coming out of Baltimore to Phoenix, I have to fly northwest for about five minutes and then turn way south about 80 degrees to avoid Dulles traffic. Then after a minute, I am pressing the no-fly space over D.C. and must again turn northeast. Once past Washington, I am stuck on narrow routes feeding this congested area. Upon reaching the middle of the US, I can start whining for direct "Zuni," the start of the arrival route into Phoenix, and I usually get it. This is why thunderstorms throw monkey wrenches into airspace back east when they push us off our regular routes. It's crazy enough when the weather is OK. Close down a few highways in the sky, and it requires improvising new routes. The controllers keep everyone separated, but they can't take the volume they can in good weather. That is why flights can be delayed when bad weather strikes a region, even though a specific airport may be unaffected by that weather.
Still, it sure beats driving!