This hoary old issue keeps coming around................
Consider this: RVR issued of less than 75m - very unlikely, HOWEVER why has a value of 75m been derived? It is to allow the aircraft to vacate the runway VISUALLY
plus, more importantly, to give the fire services sufficient visibility to find their way to any airfield incident. Now, if the fire srevices cannot guarantee a response due to low vis. then it is highly questionable whether one is now operating with legal fire cover!
I have tried in the past to extablish this minimum visibility for fire services but no one seems to have a definitive answer. This the reason why CATIIIC pioneered by BEA on the Trident never caught on. (CATIIIC = true 0/0 ops.)