AN ACR LGT PIC TAXIES HIS ACFT OFF THE TXWY ON THE WAY BACK TO THE TERMINAL DURING HVY FOG CONDITIONS.

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AFTER 2 HRS OF WAITING TO TKOF DUE TO LOW RVR ON RWY; 800-1200 FT; WE TAXIED BACK TO THE GATE FROM RWY 35L IN COS. WE WERE CLRED RWY 35L TO TXWY G AND THEN TO THE GATE. DUE TO THE EXTREMELY LOW RVR ON TXWY I ELECTED TO GO FROM TXWY G TO TXWY P LEADING TO RAMP. THIS WAS THE FIRST TXWY WE COULD USE AND I WANTED TO GET TO THE TERMINAL AREA TO USE THE BETTER LIGHTING. IT WAS EXTREMELY HARD TO FIND THE TURN INTO THE TXWY. AFTER COMING ABEAM THE TXWY P SIGN ON MY L SIDE AND WHAT APPEARED TO BE BLUE TXWY SIDE LIGHTS; I TURNED R AND LOOKED STRAIGHT DOWN TO FIND THE TAXI LINE. MY COPLT LOOKED DOWN AT THE RWY CHART TO SEE THAT WE WERE IN THE CORRECT AREA. HE LOOKED UP JUST AS MY NOSE GEAR LIGHT ILLUMINATED THE GRASS. IT WAS TOO LATE FOR BRAKING. THE GRASS AREA WAS FROZEN AND WE DID NOT BOG DOWN. I STILL SAW WHAT APPEARED TO BE TAXI LIGHTS ON BOTH SIDES. I LET THE MOMENTUM CARRY US ACROSS TO A PARALLEL TXWY THAT WAS UNCHARTED AND TEMPORARILY LIGHTED. I THEN TAXIED TO THE GATE; INSPECTED THE ACFT (NO DAMAGE) AND CALLED THE ARPT SUPVR TO TAKE ME OUT TO THE TXWY AND INSPECT FOR DAMAGE. THERE WAS NO DAMAGE TO LIGHTS; ETC. I THEN NOTIFIED CHIEF PLT AND MAINT. THE ACFT WAS INSPECTED BY MAINT AND NO DAMAGE WAS FOUND. IT WAS VERY DARK AND VERY LOW VISIBILITY. THE TEMPORARY LIGHTING GAVE THE ILLUSION OF A TXWY (SEE DIAGRAM). I STARTED THE TURN BEFORE PICKING UP THE YELLOW TAXI LINE AND THE TXWYS WERE ICY. SUPPLEMENTAL INFO FROM ACN 392896: IF I FIND MYSELF IN THIS SIT I WOULD RECOMMEND STOPPING THE ACFT AND ASKING GND TO SEND OUT A TRUCK TO GUIDE US BACK TO GATE. I ALSO FEEL THERE SHOULD BE MINIMUM VISIBILITY REQUIREMENTS FOR TAXI.