FLC OF LGT OVERSHOT THE ASSIGNED TXWY AFTER TURNOFF FROM LNDG RESULTING IN A GND CONFLICT WITH ANOTHER ACFT.

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UPON TAXI OFF RWY 12 AT MIA ON TXWY V; GND CTL CLRED US FOR A L TURN ON TXWY Q. APCHING AREA WHERE THESE 2 TXWYS AND TXWY P MERGE; LOOKED FOR SIGN INDICATING 'Q.' FROM OUR POS; THE CORRECT TXWY INDICATED BY THE SIGN WAS UNCLR. IT WAS DIFFICULT TO DETERMINE WHAT TXWY WAS 'Q' AND WHAT WAS 'P.' I STOPPED THE ACFT AND DISCUSSED WITH THE FO WHICH WAS WHICH. HE SAID TWICE AND I FINALLY AGREED THAT THE SECOND L TURN WAS 'Q.' WE BEGAN TO MOVE FORWARD SLOWLY. JUST PRIOR TO ME MOVING ONTO THE TXWY WE THOUGHT WAS 'Q;' THE FO CALLED OUT 'STOP!' WE BOTH RAPIDLY APPLIED BRAKES AND THE ACFT STOPPED IMMEDIATELY. I THEN OBSERVED A DASH 8 TURBOPROP MOVING AT A HIGH TAXI SPD FROM R TO L ON THE TXWY WE THOUGHT WAS 'Q.' HOWEVER; IN FACT; THAT TXWY WAS 'P.' THE DASH 8 CONTINUED STRAIGHT DOWN THE CTRLINE OF THE TXWY PASSING WELL CLR OF OUR ACFT (ESTIMATE 75-100 FT). WE INFORMED ATC THAT WE HAD MISSED 'Q.' THE GND CTLR ISSUED FURTHER TAXI INSTRUCTIONS AND INDICATED WE SHOULD HAVE TURNED ON 'Q' NOT 'P.' NO OTHER COMMENTS WERE EXCHANGED AND WE COMPLETED OUR TAXI IN. CONTRIBUTING FACTORS WERE: DUSK. VERY UNCLR TXWY MARKING. NEW CAPT. HIGH SPD OF OTHER TAXIING ACFT. LESSONS LEARNED: IF UNSURE OF ANY TXWY; DOUBLE-CHK ALL AVAILABLE CHARTS. DO NOT TRUST TXWY MARKINGS. ALL ACFT SHOULD TAXI AT SLOWER SPDS.