B737-800 CREW WAS CLRED AND ACCEPTED A VISUAL APCH. TWR THEN INFORMED CREW OF REDUCED RVR READINGS.

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APCHING SLC IN RECEIPT OF ATIS INFO; THE RPTED ARPT VISIBILITY WAS 4 MI. FROM 8-10 MI N; THE ARPT WAS IN SIGHT; AS WERE NUMEROUS OTHER INBOUND ACFT. THE APCH WAS BRIEFED FOR CAT I/VISUAL POSSIBILITIES. ON DOWNWIND VECTOR APCH; CTLR REQUESTED WE RPT THE ARPT IN SIGHT. SINCE THE FO WAS THE PF; I INFORMED HIM I HAD THE ARPT IN SIGHT. BY AGREEMENT WE DID NOT CALL THE ARPT IN SIGHT TO ATC UNTIL ON L BASE; OUTSIDE FLAGG WHEN WE BOTH COULD SEE RWYS; OTHER TFC AND TERRAIN IN DISTANCE. APCH CTL THEN CLRED US FOR VISUAL APCH TO RWY 34L WHICH WE HAD INSIGHT. AS THE PF (FO); CALLED 'GEAR DOWN; FLAPS 15 DEGS; SET SPD.' TWR CLRED US TO LAND ON RWY 34L; THEN ADDED 'RWY 34L RVR GREATER THAN 6000 FT; MID FIELD 400 FT.' SINCE WE WERE ALWAYS CLR OF CLOUDS; INFLT VISIBILITY GREATER THAN 3 MI WITH OUR MOST RECENT ATIS RPTING 4 MI ARPT VISIBILITY; I ACCEPTED LNDG CLRNC. THE FINAL APCH AND LNDG WERE CONDUCTED IN VISUAL CONDITIONS. ONLY TAXIING TO CLR THE RWY DID WE ENCOUNTER A BRIEF AREA OF GND BASED OBSCURATION THAT REDUCED VISIBILITY BRIEFLY. IN RETROSPECT; I QUESTION APCH CTL GIVING US A VISUAL CLRNC IF GND FOG WAS A KNOWN FACTOR. IT WAS NOT APPARENT TO US AT ANY POINT UNTIL TAXI SPD. SHOULD THIS OCCUR AGAIN; I WOULD CALL FOR A GAR TO CLARIFY WX. IF THE ATIS HAD IN FACT CHANGED AND ARPT VISIBILITY WAS BELOW 3 MI THAT SHOULD HAVE BEEN BROADCAST BY EITHER APCH OR TWR -- NOT JUST RVR GREATER THAN 6000 FT. CLRING THE RWY IT WAS EASY TO MISS THE TURN ON TXWY A AND CONTINUE TO TXWY B; THE UNLIT PARALLEL TXWY; SINCE GND CTL'S ONLY INSTRUCTIONS WERE 'TAXI E TO GATE.'