Radar doesn't guarantee any separation.
It doesn't return altitude (that comes from a transponder; secondary radar) so
- if you are in CAS whose base doesn't reach the ground, ATC must assume that a primary-only blip is below CAS, but occassionally it isn't because the plane is busting CAS
- if you in CAS whose base does reach the ground, that is slightly better because ATC can then assume he is busting, but if he is low enough he will be seen intermittently at best, and watching ATC tape replays makes it obvious that some busts are totally missed (especially a small plastic plane)
- if you are OCAS, a lot of the time:
a) no radar service
b) radar service but a lot of conflicting traffic is not reported to you (no obligation to do so, evidently, because "controller workload" is always a defence)
c) radar service is meaningless against non-Mode C/S targets (no altitude info)
etc etc etc
"Total Safety" due to radar is an illusion. Radar just improves the odds.
If you REALLY want to improve your odds against a midair, do the following (in order of decreasing importance):
1) Fly well above 1000ft and preferably above 2000ft
2) Fly in IMC
3) Fly on bad VFR weather days (you also get much better radar service
).