FLC OF ACR SMT ACFT INADVERTENTLY TAXIED OFF OF TAXIWAY ONTO A CONSTRUCTION ROAD ACCESS AT NIGHT DUE TO SNOW ON ACTUAL TAXIWAY AND MARKINGS.

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HOME ARPT IS UNDERGOING EXTENSIVE CONSTRUCTION AND REROUTING OR TAXIWAYS IN ANTICIPATION OF OPENING NEW TERMINAL BUILDING. LCL AREA HAD RECEIVED 10 INCHES OF SNOW PRECEDING 24 HRS; PLOWING HAD NOT REMOVED MUCH OF SNOW COVER. TAXIED SLOWLY TO FOLLOW WINDING TAXI RTE FROM SW RAMP IN THE DARK. SNOW OBSCURED ALL TAXIWAY BORDERS AND PAINTED LINES. SNOW CONTINUED TO FALL; OBSCURING VISIBILITY. REACHED FORK IN TAXIWAY NEAR ACFT RAMP. AREA TO R HAD NOT BEEN PLOWED RECENTLY NOR WAS IT MARKED BY TEMPORARY REFLECTIVE TAXIWAY POSTS/SIDE MARKERS. AREA TO L WAS FRESHLY PLOWED AND HARDPACKED WITH STANDARD REFLECTIVE TAXIWAY MARKERS ON EITHER SIDE OF RTE (SAME MARKERS USED AT EWR; FOR EXAMPLE). IN THE DARK WITH UNLIMITED VISIBILITY; THESE STANDARD ARPT MARKERS COMBINED WITH FRESHLY PLOWED TRACK LED US TO BELIEVE WE WERE FOLLOWING WINDING RTE OF TEMPORARY TAXIWAY. SHORTLY AHEAD I SAW CLRNC (HORIZ) FROM WINGTIPS NARROWING AND BROUGHT ACFT TO A STOP. CALLED GND CTL AND THEY CONFIRMED WE HAD DEPARTED TAXIWAY AND ENTERED CONSTRUCTION ACCESS RTE. CALLED COMPANY TO SEE IF THEY WANTED US TO SHUTDOWN AND TOW ACFT BACK TO TAXIWAY. COMPANY SENT WINGWALKER ALONG WITH ARPT AUTHORITY EMPLOYEE WHO SERVED AS WING GUIDES AS WE CREPT FORWARD TO JOIN NEAREST TAXIWAY. WE PROCEEDED TO GATE WITHOUT INCIDENT AND SHUT DOWN ENGS TO PERFORM A SECOND THOROUGH PREFLT. NO PART OF AIRFRAME OR APPURTENANCES CAME IN CONTACT WITH SNOW DRIFTS AND NO DAMAGE NOTED. SIGNED CREW AIRWORTHINESS CHK BOX IN ACFT LOG AND PROCEEDED TO FIRST DEST ARPT WITH NO DIFFICULTY. IN DAYLIGHT; PERFORMED ANOTHER VISUAL INSPECTION TO MAKE CERTAIN ACFT UNTOUCHED -- ALL OK. PROVIDED CHK PLT WITH ORAL RPT PER COMPANY POLICY.