I have just seen a documentary on Tv investigating the problems with the "see and be seen" method of traffic avoidance and the development of TCAS.
A live demonstration of the difficulties of seeing fast moving aircraft was demonstrated by test pilots flying a Sabreliner.
However, a tragic occurrence. Southern California has had its share of mid-airs.
The documentary had a look at a fatal mid-air some years ago, also in the San Diego area.