PLT TAKES OFF ON TXWY AT COS.

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I HAD LANDED AT COS TO LET A FRIEND OFF THE ACFT FOR A VISIT WITH HER FAMILY. MY WIFE AND I THEN CONTINUED ON TO APA. I HAD LANDED ON RWY 17R UPON ARR AND PARKED ON THE GA RAMP. FOR DEP I WAS CLRED TO TAXI TO RWY 17R VIA TXWY A. I WENT TO THE E EDGE OF THE RAMP; PICKED UP THE YELLOW LINE AND TAXIED N. I HAD ONLY LANDED AT COS ONE OTHER TIME -- ABOUT 12 YRS AGO; AND SO WAS NOT FAMILIAR WITH THE ARPT. GOING N ALONG THE RAMP I THOUGHT I WAS ON TXWY A. AT TXWY A2; I ASKED FOR AN INTXN TKOF. GND CTL SAID; 'INTXN TKOF APPROVED; HOLD SHORT OF RWY 17R AT TXWY A2.' I TURNED E ON TXWY A2 AND STOPPED SHORT OF TXWY A; THINKING I WAS SHORT OF RWY 17R. WHEN I RECEIVED TKOF CLRNC; I TURNED ONTO TXWY A AND MADE THE TKOF. AFTER I WAS IN THE AIR; THE TWR ASKED ME TO CONFIRM THAT I HAD DEPARTED THE TXWY. FACTORS IN PROB: 6000 HR PLT WHO THINKS HE IS A PROFESSIONAL; BUT DID NOT TAKE TIME TO STUDY THE ARPT DIAGRAM. AT A2 INTXN WITH TXWY A I DID NOT SEE A 'HOLD SHORT LINE' OF COURSE BUT THIS DID NOT WARN ME. WHEN I TURNED ONTO TXWY A FOR TKOF; I SHOULD HAVE REALIZED THERE WAS NO WHITE LINE DOWN THE CTRLINE AND THAT THERE WERE BLUE LIGHTS ALONG THE EDGES. IT ALL ADDS UP TO BEING TOO COMPLACENT AND TO NOT PAYING ATTN. LUCKY FOR ME THERE WAS NO REAL PROB AS A RESULT.