For most bizjets and regional airliners, they have TCAS [0]. The rule is that pilots follow TCAS resolution, even if the controller gives them contradictory instructions.
There has only been one crash[1] where TCAS was used on both aircraft, and functioning. One of the aircraft followed the controller, ignored the TCAS resolution and a mid-air collision occurred.
There has only been one crash[1] where TCAS was used on both aircraft, and functioning. One of the aircraft followed the controller, ignored the TCAS resolution and a mid-air collision occurred.
[0] https://en.wikipedia.org/wiki/Traffic_collision_avoidance_sy...
[1] https://en.wikipedia.org/wiki/%C3%9Cberlingen_mid-air_collis...