Tough one...safe lateral separation depends on so many factors. Some answers could be found in ICAO Annex 11 ,Attachment A -Material related to a method of establishing ATS routes defined by VOR ,and Attachment B- Method of establishing ATS routes for use by RNAV equipped aircraft...also try to find ICAO Doc. 9869 -Manual on Airspace Planning Methodology for the Determination of Separation Minima...