Quote Phil from the USA: "Kind of curious as to the RA criteria in terminal airspace. Here it is pretty routine to have 500 foot separation between VFR piston and transports".
Two of my four TCAS RAs were in the airspace just to the North East of New York where you tend to cruise for an extended time at 16,000' in a Victor airway in just those circumstances. IFR traffic mixing it with VFR traffic at 500' levels. One RA against a twin coming the opposite way at 500' lower ended up with us in a climb and triggering another RA in an aircraft above us. The controller asked us why we had responded to the RA when we should have seen it would cause problems!
And before the usual US posters point out that it has always been that way and if we can'r hack it, we should stay out of the kitchen - or some other comment, I would like to point out that as a UK operator, we had no choice but to respond.