FLC OF BE20 AIR AMBULANCE TAXIES ON CLOSED TXWY WHERE CONSTRUCTION IS IN PROGRESS. REQUEST TWR TO ALLOW A TURN TO EXIT TXWY.

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ON LNDG ROLLOUT WE WERE INSTRUCTED TO EXIT ON TXWY R OR D? POSSIBLY THE CTLR SAID DELTA 4. AFTER SLOWING THE ACFT WE SPOTTED THE TXWY R SIGN AND FOUND A YELLOW LEAD-IN LINE. WHEN WE EXITED THE RWY WE NOTICED THAT A CONSTRUCTION CREW WOULD BLOCK OUR EXIT A LITTLE FURTHER DOWN. WE HAD MISTAKEN THE NEXT PREVIOUS TXWY 'D6' FOR TXWY R ('THEY ARE VERY CLOSE TOGETHER') IN THE DARK. AT THE TIME WE WERE EXITING WE HONESTLY BELIEVED THAT WE WERE ON TXWY R AND THE PNF EVEN CALLED 'CLRING AT TXWY R.' WE RECOGNIZED THE MISTAKE AND ASKED THE TWR FOR CLRNC BACK ONTO RWY 18L AND TOOK THE NEXT TXWY WITHOUT INCIDENT OR CONFLICT. FACTORS: DARKNESS AND DIFFICULTY IN FINDING THE CORRECT YELLOW LEAD-IN MARK. A GREEN HIGH SPD RWY LIGHT SYS WOULD HAVE PREVENTED THIS. AUTOMATED ATIS READ NUMEROUS TXWY CLOSURES AT A HIGH RATE OF SPD THAT MADE IT DIFFICULT TO CATCH EVERYTHING. MOST OF THE CLOSURES HAVE BEEN IN EFFECT FOR A LONG TIME AND THE TXWY 'D6' WAS NEW AND CAUGHT US BY SURPRISE. CREW FATIGUE -- XX00 AT NIGHT ON THE LAST LEG. HOME COMPANY POLICY HAS EACH PLT ON THE DAY/NIGHT SHIFT ON A WEEKLY ROTATIONAL BASIS. IT IS DIFFICULT TO ADJUST TO THE NIGHT SCHEDULE.