TCAS is mostly "how close to another plane am I?"

In flight, the answer really shouldn't ever be "less than 500 feet". During landing, the answer almost certainly will be "less than 500 feet"; a plane's queued up to enter the runway after you land, a ground vehicle is working on something near but not on the end of the runway, etc.

It's a surprisingly tough challenge to solve a) reliably and b) in a way that doesn't cause a whole bunch of false go-arounds wreaking havoc on the busy airport.