Do modern commercial aircraft include radar to detect traffic in their vicinity or perhaps have some mechanism to receive transponder signals.
I know they have forward facing weather radar but the ability to detect and be warned of other traffic on a collision course would seem to be a good idea.
I'm particularly reminded of the Proteus Airlines Flight 706 incident where a mid-air collision occurred because of visual blind-spots and the ATC not knowing the other aircraft was even out there. I am also aware in this case a transponder monitor would not have helped either...
I suppose TCAS does do the transponder bit, but wont detect aircraft without the necessary device turned on. I'm more thinking of a short range RADAR or even LIDAR, something akin to, though longer range than, the things they are hooking up to cars these days.