AN MD80 FLC TAXIED UPON AN ACTIVE RWY WITHOUT CLRNC AT MIA.

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FIRST FLT OF THE DAY. WE PUSHED FROM GATE AND WERE CLRED TO EXIT THE RAMP AT TXWY T; TURN R ON TXWY Q AND TAXI TO RWY 9L. THE CAPT WAS VERY EXPERIENCED (30 YRS AIRLINE EXPERIENCE) AND COMPETENT; AND HE WAS FAMILIAR WITH THE ARPT. ON TOP OF THAT; HE ASKED ME TO REVIEW MY ARPT DIAGRAM AND WE AGREED UPON THE TAXI RTE BEFORE HE MOVED THE ACFT. BECAUSE OF ALL OF THAT CAUTION; I WAS VERY COMFORTABLE THAT HE KNEW WHERE HE WAS GOING. AS HE WAS EXITING ON TXWY T; HE CALLED FOR THE TAXI CHKLIST. I AM NEW TO THE ACFT (FIRST TRIP AFTER IOE) AND PUT MY HEAD DOWN TO DO THE FLOWS AND PICK UP THE CHKLIST TO READ. WHEN I LOOKED UP; HE HAD PASSED TXWY Q AND WAS ABOUT TO ENTER ONTO RWY 12/30. STARTLED; I SAID 'YOU ARE GOING TO CROSS A RWY?' IN A HALF QUESTION; HALF STATEMENT TONE OF VOICE. HE STATED THAT WE WERE CLRED TO TAXI TO THE RWY. AT THAT MOMENT GND CALLED AND SAID THAT WE WERE ENTERING A RWY; TO TURN R ON TXWY S AND GO TO TXWY Q. HE CLAIMED THAT THE STRANGE ANGLES OF THE TXWYS AND THE TXWY MARKERS ARE WHAT CONFUSED HIM; AND I CAN SEE WHY IN HINDSIGHT. I COULD HAVE PREVENTED THIS FROM HAPPENING. I SHOULD NOT HAVE LOOKED DOWN UNTIL HE MADE THE CRITICAL TURN; AND I SHOULD HAVE NOT DIVERTED MY ATTN FOR MORE THAN A FEW SECONDS AT A TIME. ALSO; I LOOKED UP AND REALIZED THAT SOMETHING WAS NOT RIGHT BEFORE HE ACTUALLY ENTERED THE RWY. I COULD HAVE SAID 'STOP;' OR EVEN STOPPED THE ACFT MYSELF IN TIME IF I HAD REACTED INSTANTLY. WITH 1 OR 2 SECONDS OF INDECISION; IT WAS TOO LATE.