B737 FLT CREW BECOMES CONFUSED WITH SIGNAGE AND TAXIED ACROSS ACTIVE RWY AT MIA.

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DUE TO SHORT TAXI FROM RAMP AREA TO DEP RWY 27L COCKPIT WAS BUSY DOING CHKLISTS AND SWITCHING RADIO FREQS. ACFT WAS CLRED INTO POS AND HOLD ONTO RWY 27L. CAPT CALLED FOR TKOF CHKLIST WHICH FO PROCEEDED TO DO. AS CAPT TAXIED ONTO RWY 27L; HE SAW DIRECTLY AHEAD OF HIM SIGNS FOR RWY 30; WHICH WAS SOME 500 FT AHEAD. CAPT WAS MOMENTARILY CONFUSED ABOUT HIS ASSIGNED DEP RWY AND CROSSED RWY 27L ONTO THE TXWY BTWN RWYS 27L AND RWY 30. TWR CALLED AND SAID WE WERE HEADED FOR RWY 30 AND HE COULD LET US TAKE OFF ON THAT RWY IF WE DESIRED. WE CHKED OUR PAPERWORK; SAW THAT WE WERE OK FOR RWY 30 DEP. TWR THEN CLRED US FOR TKOF ON RWY 30. THERE WAS NO INCURSION ON RWY 30 PRIOR TO OUR BEING CLRED ONTO THAT RWY. THE BUSY COCKPIT; AND THE DISTR OF THE FO COMPLETING THE TKOF CHKLIST; TOOK THE FO OUT OF THE LOOP AS THE CAPT TAXIED ONTO THE RWY. THE CAPT MAY HAVE HEADED FOR RWY 30 BY ACCIDENT; PARTIALLY BECAUSE OF THE SIGNAGE; BUT ALSO; HE HAD LANDED ON RWY 30 SOME 30 MINS EARLIER; SO IT MAY HAVE BEEN IN HIS HEAD. ANOTHER FACTOR MAY HAVE BEEN THE ANGLE THE ACFT WAS ON AS IT CAME TO RWY 27L. IT WAS NOT PERPENDICULAR; BUT AT AN ANGLE SO THE CTRLINE LIGHTS WERE NOT NOTICEABLY VISIBLE.