I initially rejected an edit to this question as it created an extra traffic-separation tag for a question already tagged air-traffic-control. Since the primary function of ATC is traffic separation, wouldn't an air-traffic-control tag be sufficient? The edit has since been approved by others.
Well, traffic separation involves more than just air traffic control. VFR traffic is responsible for their own separation, and many times they aren't even talking to ATC. There are also technologies that are strongly linked to traffic separation (TCAS, ADS-B, etc.). That being said, I'm not sure that there will be a lot of people searching for "traffic separation" type questions. More likely, they will be searching for questions about ATC rules, technology questions, etc. that involve it.
I could go either way on this one. As a tag, I doubt that there will be a lot of activity so we probably don't need it, but it doesn't really hurt either. I say let it go and we'll see if there are more questions down the road, and if not we can get rid of it then.