A long time ago I was involved with OPS/AFS trial to determine a practical minimum visibility for operations (BLEU Bedford). This involved a simulated off-the-side runway accident which was used to develop search techniques, communications, and the most suitable vehicles (seat height w.r.t. fog thickness/depth).
The results indicated that a met vis of 60m was an absolute practical minimum; it was assumed that surface radar would not available. This visibility translates into different RVRs depending on day/night and fog type. Thus a 75m limit seems very sensible.
Subsequently during an operation in 150m RVR night at GLA, the turn-round coffee-stop was marked by the absence of travelers, staff, and general hub-bub. This was explained by the met vis being between 40-60m and the bus system and taxis had all given up!