TXWY EXCURSION EXPERIENCED BY ACR MLG.

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AFTER LNDG ON RWY 12R AT HOU TWR INSTRUCTED US TO TURN L AT TXWY L; CROSS RWY 12L (WHICH WAS CLOSED) AND CONTACTED GND. I COMPLETED THE FLOW FOR THE AFTER LNDG CHK AND WAS IN THE PROCESS OF SWITCHING TO GND. WHILE TURNING FROM TXWY L TO TXWY C THE ACFT NOSE SLID TO THE L AND TILTED SOMEWHAT. I WAS IMMEDIATELY AWARE THAT WE HAD GONE OFF THE TAXI SURFACE BUT WAS VERY SURPRISED. WE WERE TAXIING IN A VERY NORMAL AND SAFE MANNER; AT THE PROPER SPD OUR NOSEWHEEL ON THE CTRLINE. PA FROM CAPT TO REMAIN SEATED. IMMEDIATE VISUAL INSPECTION BY ME FROM PAX WINDOW CONFIRMED NO DANGER TO PAX; CREW OR ACFT. WE CALLED TWR AND COMPANY AND COMPLETED AFTER LNDG AND SHUTDOWN CHKLISTS. APU AND PACKS RUNNING. BRIEFED THE #1 FLT ATTENDANT. WENT TO MAIN CABIN TO CHK PAX WELL-BEING. OVERALL; WERE CALM; PATIENT; AND ORDERLY. ALL 90 PAX WERE OFF-LOADED WITHIN AN HR BY AFT STAIRS AND BUSSED TO TERMINAL. COMPLETED EXTERIOR WALK AROUND AND FOUND NO DAMAGE EXCEPT POSSIBLE TIRE ABRASIONS TO L MAINS. ACFT WAS RESTING WITH THE L MAIN JUST OFF THE HARD SURFACE ON THE W CORNER OF TXWY L AND TXWY C; TILTED SLIGHTLY TO THE L. OBSERVATION: BEFORE INCURSION PERSPECTIVE WAS NORMAL FROM COCKPIT. HOWEVER; THE CTRLINE AT THE INTXN OF TXWY L AND TXWY C AT HOU WILL NOT ALLOW PROPER CLRNC FOR ACFT. LINES ARE PAINTED IN A GRADUAL ARC. POSSIBLE FIX WOULD BE A 90 DEG INTERSECT FOR TAXI LINES AND NOTE IN EAB. SUPPLEMENTAL INFORMATION FROM ACN 260336: THE ACFT INVOLVED WAS AN MLG WHICH HAS A WHEEL BASE OF APPROX 75 FT. TXWY L; E OF RWY 12R DRAWS DOWN TO 45-50 FT WIDE AT THE POINT JUST PRIOR TO THE TURN. AS A CREW WE COULD NOT HAVE OPERATED ON THIS TXWY IF WE HAD BEEN AWARE IT WAS 45-50 FT WIDE AT THIS POINT. SUGGEST THAT FOR PART 121 ARPTS THE ARPT PLATE INCLUDE AN ARPT 'STANDARD' TAXI WIDTH; AND THAT ANY TXWY THAT HAS A LETTERED NAME THAT IS LESS THAN THE 'STANDARD' WIDTH BE ANNOTATED ON THE ARPT DIAGRAM. WITHOUT THIS INFO THERE IS NO WAY TO BE CERTAIN OF THE WIDTH EXCEPT WITH THE NAKED EYEBALL SUBJECT TO THE CONSTRAINTS OF NIGHT; RAIN; LOW VISIBILITY; ETC. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: CALL TO THE ARPT REVEALS THAT THE TXWY IS 50 FT WIDE WHEREAS THE OTHER TXWYS ARE 75 FT WIDE. THE ARPT HAS ISSUED A NOTAM PROHIBITING ACR OPS ON TXWY L BTWN RWY 12L AND TXWY C UNTIL THE PROB CAN BE SOLVED.