TBL 5-4-1 in the Aeronautical Information Manual (AIM) provides conversion from RVR to visibility in statute miles. I've reversed the first two columns from the AIM table to allow for comparison of the listed RVR to the actual distance in resulting from multiplying 5,280 by these fractions.
Why would there would be a difference of more than 100 feet? I understand that the RVR equipment may not be accurate enough to measure a value like 1,320, but why use 1,600 rather than 1,300 or 1,400?
Furthermore, the lack of RVR actually allows flight in lower true RVR for the 1600/quarter mile level. Let's say that the RVR was 1500 on an approach with mins of 1600. If suddenly the RVR equipment failed and a report 1/4 statute miles was substituted the approach would again be available. Usually the failure of a more precise piece of equipment substituted for a less precise calls for added margin of safety. Why the opposite approach here?
SM RVR ACTUAL - - - 1/4 1600 1320 1/2 2400 2640 5/8 3200 3300 3/4 4000 3960 7/8 4500 4620 1 5000 5280 1 1/4 6000 6600