I believe the 75m RVR limit was introduced as a consequence of the crash in thick fog of BEA Viscount G-AOHU on 7th Jan 1960 at LHR. A heavy landing collapsed the nose gear, causing the aircraft to catch fire as it slid down the runway. The fire service took about 30 mins to find the aircraft, by which time it had burned out. By great good fortune, no lives were lost. But as previous posters have alluded, it was accepted that 75m was a minimum acceptable for rescue operations. I once met a survivor of this accident whose description of the event was terrifying.