TCAS comes in two flavors; TCAS I, and TCAS II. TCAS II provides resolution advisories, whereas TCAS I does not.
Even a professionally installed TCAS II system in a radar environment has it's limitations. It's a tool, but still misses traffic, and collisions still occur with TCAS onboard and functioning.
See and be seen is still the best policy, with everything else serving only to back that up.