A C172 SINGLE PLT INCURRED ON RWY 21R AT LUK.

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I WAS DEPARTING LUNKEN ARPT FROM THE TERMINAL AREA. ATIS WAS RPTING WINDS AND ALTIMETER; AND ALSO PROVIDED THE STANDARD PHRASE USED AT LUK -- 'READ BACK ALL RWY HOLD-SHORT INSTRUCTIONS.' AFTER CALLING GND CTL REQUESTING A NORTHERLY DEP; I RECEIVED THE FOLLOWING INSTRUCTION: '...TAXI TO RWY 21R.' (NO MENTION WAS MADE OF HOLD-SHORT.) I PROCEEDED TO TAXI NE ON TXWY A; CROSSED RWY 21R; TURNED L ON TXWY G; AND PERFORMED MY RUN-UP ON THE PAD. AFTER A NOMINAL RUN-UP; I CALLED TWR AND WAS CLRED TO TAXI INTO POS ON RWY 21R AND HOLD; PENDING ARR OF TFC ON RWY 25. AFTER THAT ACFT'S ARR AND XING THROUGH THE INTXN WITH RWY 21R; I WAS CLRED FOR TKOF AND THE FLT PROCEEDED TO A SAFE CONCLUSION. HERE IS THE REASON I AM SUBMITTING THIS ASRS RPT: UPON RETURN HOME THAT NIGHT; I CHKED THE AIM AND RE-LEARNED THE FACT THAT '...THE ABSENCE OF HOLDING INSTRUCTIONS AUTHORIZES THE ACFT TO 'CROSS' ALL RWYS WHICH THE TAXI RTE INTERSECTS EXCEPT THE ASSIGNED TKOF RWY.' MY MISTAKE WAS THAT I CROSSED RWY 21R WITHOUT CLRNC TO DO SO. I WOULD LIKE TO MAKE THE FOLLOWING SUGGESTION: EVEN THOUGH SOME WOULD CONSIDER THIS REDUNDANT; WHY NOT HAVE THE CTLR ISSUE THE FOLLOWING CLRNC TO TAXI: '...TAXI TO RWY 21R; HOLD SHORT OF RWY 21R ON TXWY A.' THIS WOULD BE UNAMBIGUOUS; AND CONFIRM THE FACT THAT YOUR TAXI INSTRUCTIONS REQUIRE YOU TO CROSS THE ACTIVE RWY WHICH YOU CURRENTLY (AT THE TIME OF THE TAXI CLRNC) DO NOT HAVE PERMISSION TO CROSS. THIS IS ESPECIALLY TRUE IN LIGHT OF THE ADMONISHMENT TO 'READ BACK ALL RWY HOLD SHORT INSTRUCTIONS' IN THE ATIS BROADCAST. NO MENTION OF THE 'HOLD SHORT' ISSUE IN THE TAXI INSTRUCTIONS; AS WAS THE CASE WITH ME; COULD LEAD PLTS TO ASSUME THAT THEY'RE CLRED ALL THE WAY TO THE DEP END OF THE RWY IF THEY DIDN'T RECALL THE AIM PROC.