The methods for measuring RVR and prevailing visibility are quite
different.
RVR is measured in a specific direction, usually using transmissiometers, while the prevailing visibility will be
the average and, if less than 1,500m, the minimum
value in a specific direction.
This means that there can be no direct comparison and RVR is likely to
be the same as or greater than the lowest reported value of prevailing
visibility.