Hey Hey,
Here is why 600 feet RVR was set up here this end. (from what i remember) The foggest area in eastern Canada happens to be the airport and surrounding areas. A local operator indorsed their crews on the sim for 600 feet not meters RVR take off only. This would allow the helos departure providing some allowences were made: cat 2 runway with centre line lights and a alternate airport for emergencies with weather above IFR limits within 30 minutes at single engine cruise.
This method was not used alot but at times it came in handy.
2dalou