I've seen that one too (in a different place, obvs). "Live" traffic? A lot of it seems pretty dead!
"Live traffic" seems to be a $kyride-ism.
No "Live traffic" or "Live lane" is fairly standard highways engineering language for an operational lane rather than say the hard shoulder or a closed lane for roadworks. However I would say that while that sign makes perfect sense to the engineers and workmen on the site they have not passed it through a plain english or general population filter to take the jargon out.
That would explain why the $kyride people use it, given that the event is planned around road closures. I hadn't come across the term other than having dire warnings (to the effect that the road was about to turn into a pumpkin and I should immediately stop cycling on it to avoid splatty
DETH) that used the phrase shouted at me through megaphones.
The implication that cyclists aren't live traffic in the sign Ham posted above probably makes sense in context (I'm imaging some closure to general traffic that cyclists are able to bypass via a shared-use footway or something), but surely "cyclists rejoin carriageway" or "cyclists merge with traffic" would be more standard.