UNAUTH UNCOORD RWY ENTRY XING IN A NIGHT OP. RPTR SUGGESTS THAT ARPT PROB TXWY MARKING GUIDANCE WAS A FACTOR.

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WE WERE CLRED BY MEMPHIS GND CTL TO TAXI FROM CARGO RAMP BY 'TXWY C TO RWY 18L.' BEING UNFAMILIAR WITH MEMPHIS; I DOUBLECHKED WHAT I NEEDED TO BE LOOKING OUT FOR TO TURN R FROM TXWY C TO RWY 18L THRESHOLD. I DETERMINED FROM THE ARPT DIAGRAM THAT I NEEDED TO LOOK FOR TXWYS M AND B. WHILE TAXIING N ON TXWY C; BOTH FO AND I MISSED TXWYS M AND B. WE DID NOT REALIZE THIS AT THE TIME AND CONTINUED N ON TXWY C. UPON REACHING RWY 9/27; WE VISUALLY CLRED OURSELVES AND CROSSED RWY 9/27. AFTER XING; I REALIZED WE MUST HAVE GONE TOO FAR N ON TXWY C; STOPPED THE ACFT; AND WE CALLED GND CTL TO TELL THEM WE WERE LOST. THEY GAVE US PROGRESSIVE TAXI INSTRUCTION TO RWY 18R. CAUSES: 1) INADEQUATE SIGNS/MARKINGS TO INDICATE TXWYS M AND B TO ACFT TAXIING N ON TXWY C. 2) INADEQUATE SIGNS/MARKINGS TO INDICATE DIRECTION TO THRESHOLD OF RWY 18L TO ACFT TAXIING N ON TXWY C. 3) ATC PHRASEOLOGY USED TO GIVE TAXI CLRNCS TO ACFT 'TO TAXI TO RWY 18L.' I UNDERSTOOD THIS TO MEAN 'CLRED TO CROSS ALL RWYS AND TXWYS' BTWN ME AND RWY 18L. THIS IS WHY I DID NOT SEEK SPECIFIC CLRNC TO CROSS RWY 9/27 WHEN I REACHED IT. NOT REALIZING THAT RWY 9/17 WAS NOT BTWN THE CARGO RAMP AND RWY 18L; I ASSUMED I HAD ALREADY BEEN CLRED TO CROSS IT. 4) NIGHT; DARK CONDITIONS. 5) UNFAMILIAR WITH ARPT LAYOUT. 6) FATIGUE. SOLUTIONS: 1) LIGHTED SIGNS/MARKINGS AS DESCRIBED IN 1) AND 2) ABOVE. 2) CHANGE ATC PHRASEOLOGY AND MEANING OF 'CLRED TO RWY XYZ.; THIS SHOULD NEVER IMPLY ACFT IS CLRED TO CROSS ANY RWY. AS IS STANDARD PROC NOW; THIS PHRASE SOMETIMES DOES AND SOME DOES NOT MEAN PLTS CAN ASSUME RWY XING CLRNCS. THIS CAUSES UNNECESSARY CONFUSION AND UNDERMINES SAFETY. 3) PLTS SHOULD TAKE PARTICULAR NOTE AS TO WHETHER ANY RWYS LIE BTWN THEM AND WHERE THEY ARE CLRED TO GO. HAD I TAKEN INTO ACCOUNT THAT I DID NOT NEED TO CROSS ANY RWYS TO GO FROM CARGO RAMP TO RWY 18L; I WOULD HAVE ASKED FOR SPECIFIC CLRNC BEFORE XING ANY; OR I WOULD HAVE; AT THE LEAST; REALIZED MY MISTAKE BEFORE XING ANY RWYS. AS IT WAS; I WAS STILL LOOKING FOR TXWYS M AND B AND THINKING; SINCE I HADN'T YET SEEN THEM; I WAS STILL GOING IN THE RIGHT DIRECTION; WHEN IN FACT I HAD ALREADY PASSED THEM.