Preventing crashes is done by what is called "separation". This means that the aircraft are kept apart by a minimum distance at all times. Modern aircraft also have collision avoidance systems that see nearby aircraft and warn pilots if other planes are too close. This system is a backup for when air traffic control is not available or if something goes wrong with ATC. Air traffic controllers also can provide information to pilots. But pilots do not have to follow commands of ATC. They can make changes in order to keep plane safe.