Originally Posted by
Equivocal
As whowhywhen says, 2.5 mile buffer works ok if you know that traffic inside the reserved area will also stay at least 2.5 miles from the edge and I think Doc 4444 assumes that traffic on both sides are under positive radar control (and that the standard radar separation is applicable).
Indeed, not sure why that 4444 para was brought up here as I am confident most of us are proficient with fundamental ICAO procedures. As the 2.5NM in my example also apply when the R/D areas aren't directly on the border but further downstream (and also not only under vectoring) it isn't really relevant to the original question which was specifically about P/R/D areas and not separation to adjacent airspace.