There are a number of ways to provide some assurance that airplanes won't run into each other while airborne, and nearly all of them have been put to use at one time or another in the history of aviation. These days, the prevention of midair collisions—particularly those involving airliners and other high-flying jets—has become positive and proactive because of the radar control services provided by the ATC in the United States.
This sort of positive ground controlling of the flow of air traffic is the procedure being used in most locations around the world—and certainly at those places where there is a high volume of local and/or international air traffic.
A secondary backup system to the positive-separation standards of air traffic control has been the use of modern Traffic Alert and Collision Avoidance Systems (TCAS) and related electronic devices inside the cockpits of airliners, and these days, in many General Aviation airplanes. These electronic warning systems provide the pilot with a secondary line of defense from potential conflicts.
Traffic separation hasn't always been this good. In earlier years, collision avoidance was mostly handled by following basic rules. Procedures such as odd/even altitude assignments, or simply having very little traffic being spread across very large areas had been the operating methods of the day. It took a bizarre airline accident that occurred in literally the middle of nowhere to get the subject of collision avoidance started down the road toward more positive methodology.
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