B727 CAPT LEFT RWY ONTO A NON PAVED AREA WHILE TAXIING IN A SNOW STORM.

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AFTER AN UNEVENTFUL LNDG ON RWY 9R WE WERE TOLD BY ATC TO EXIT THE RWY AT TXWY M3. I SAW A SIGN FOR TXWY M3 AND STARTED TO TURN L. BY THIS TIME WE WERE WELL WITHIN TAXIING SPD FOR THOSE CONDITIONS. THE TXWY CTRLINE MARKINGS AND LIGHTING WERE COMPLETELY INVISIBLE. I SAW 2 BLUE TXWY LIGHTS AND THOUGHT THEM TO BE THE ENTRANCE FOR TXWY M3. SHORTLY AFTER; WE REALIZED WE HAD DEPARTED THE HARD SURFACE. SHORTLY AFTER THAT I RECOGNIZED THE INTXN OF TXWY M AND TXWY M3 JUST SLIGHTLY AHEAD AND TO OUR R. WE WERE VERY CONCERNED ABOUT BEING SO CLOSE TO THE ACTIVE RWY AND TXWY M3 AND SINCE THE ACFT SHOWED LITTLE RESISTANCE TO TAXIING WE JOINED TXWY M JUST W OF THE INTXN MENTIONED EARLIER. WE THEN CONTINUED TO THE GATE NORMALLY. A WALKAROUND WAS PERFORMED BY MYSELF AND THE SO; AND WE NOTICED NOTHING UNUSUAL. THE COMPANY WAS INFORMED AND THE ACFT WAS SENT TO THE HANGAR FOR MAINT INSPECTION. I'M SURE WE DID NOT COME IN CONTACT WITH ANY TXWY EDGE LIGHTS. CONDITIONS AT THE FIELD WERE VERY POOR. WINDS WERE 080 DEGS AT 32 KTS; SOMETIMES RPTED AS GUSTING TO 38 KTS. THE RVR WAS RPTED AS 2200-1800 FT THROUGHOUT THE APCH. ALTHOUGH I SUSPECT THE VISIBILITY WAS LESS THAN THAT IN THE MIDFIELD AREA. RWY APCH LIGHTS BECAME VISIBLE AT AROUND 250 FT AGL. DURING THE APCH BRIEFING; WE DISCUSSED EXITING THE RWY AT TXWY F OR TXWY M5; AS I HAD DONE MANY TIMES IN THE PAST. BUT CONSIDERING OUR MUCH REDUCED GND SPD; WE SHOULD HAVE ALSO CONSIDERED THE POSSIBILITY OF BEING ABLE TO EXIT AT TXWY M3. OUR LNDG WT WAS ABOUT 135000 LBS. SUPPLEMENTAL INFO FROM ACN 424549: IN MY OPINION; THE PRIMARY CAUSE OF THIS EXCURSION WAS THE LOW VISIBILITY AND BLOWING SNOW. THE NEARLY BLIZZARD- LIKE CONDITIONS MADE DETERMINING TXWY LOCATION DIFFICULT. TO PREVENT A RECURRENCE; I WOULD SUGGEST THAT ARPTS USE 'FOLLOW ME' VEHICLES TO GUIDE ACFT WHEN THE CONDITIONS ARE BAD.