THE FLC TAXIED THE DC9 CONTRARY TO GND CTL INSTRUCTIONS.

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RWY INCURSION OF RWY 18/36 WHILE TAXIING FOR FULL LENGTH OF RWY 23 AT BIRMINGHAM; AL. GND CLRNC WAS TO TAXI TO RWY 23 VIA TXWYS A AND A5. APCHING TXWY A5 THE FO SWITCHED TO TWR AND REMINDED ME (CAPT) THAT THE TKOF DATA WAS BASED ON FULL LENGTH. I INCORRECTLY STARTED TAXIING DOWN TXWY N. I REALIZED MY MISTAKE AND THE FO AGREED. I CALLED THE TWR AND TOLD THEM WHERE I WAS; I HAD MADE A MISTAKE; AND COULD I TURN L ON RWY 18/36 TO GET UP TO RWY 23. THE TWR ASKED IF I WAS ON RWY 18/36 YET AND I TOLD THEM I WAS JUST COMING ON TO THE RWY (WE; THE CREW HAD CLRED BOTH WAYS AND THERE WAS NO TFC ON THE RWY OR ON FINAL FOR RWY 18/36). TWR SAID TO PROCEED ACROSS RWY 18/36 ON TXWY N AND GO BACK TO GND CTL. GND CTL TOLD US TO MAKE A 180 DEG IN THE RAMP; GO BACK OUT TXWY N TO TXWY A5. WE WENT BACK TO TXWY A5; CHKED THE TKOF NUMBERS MANUALLY IN THE BOOK FOR AN INTXN TKOF FROM TXWY A5 WITH APPROX 8000 FT OF RWY. I DETERMINED WE WERE LEGAL SO WE TOOK OFF FROM THE INTXN AT TXWY A5 FOR RWY 23. COMMENTS: RWY 23 WAS THE PRIMARY RWY; NO GARS OR TFC CONFLICTS OCCURRED ON RWY 18/36. I NOTIFIED THE TWR OF MY MISTAKE AND WAS IN RADIO CONTACT WITH THE TWR AND UNDER THEIR DIRECTIONS TO CONTINUE ACROSS THEIR SECONDARY (BUT ACTIVE) RWY 18/36. TOTAL EXPOSURE TIME ON RWY 18/36 WAS ABOUT 5 SECONDS. SINCE MOST ACFT USING RWY 23 TKOF FROM TXWY A5 NO 'HOLD SHORT OF RWY 18/36' INSTRUCTIONS WERE GIVEN SINCE MY INCORRECTLY STARTING DOWN TXWY N TO USE FULL LENGTH OF RWY 23 WAS NON-STANDARD. RECOMMENDATIONS: WHEN GND ISSUES TAXI CLRNC THEY SAY 'CLRED TO RWY 23 INTXN DEP VIA TXWYS A AND A5. RWY LENGTH FROM TXWY A5 IS 8000 FT.' BIRMINGHAM PUT UP A LARGE SIGN AT ENTRANCE TO TXWY N FROM TXWY A SAYING 'RESTR TXWY' OR SOME OTHER NOTATION SO NO ONE ELSE WILL USE TXWY N TO GET TO FULL LENGTH OF RWY 23 FOR TKOF.