I think I remember seeing somewhere that since it's an observation at the surface, statute miles are used for the visibility. Would be nice if someone had a reference.
The FAA has this handy chart to decode METARs (which are the printed version of the ATIS) and TAFs.
Prevailing visibility; in U.S., Statute Miles & fractions; above 6 miles in TAF Plus6SM. (Or, 4-digit minimum visibility in meters and as required, lowest value with direction)