WDB PIC PERFORMS AN UNAUTH UNCOORD RWY ENTRY XING. RWY INCURSION.

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AN UNEVENTFUL LNDG WAS MADE ON RWY 9R AT ATLANTA HARTSFIELD INTL. AFTER EXITING THE RWY ON A HIGH SPD TXWY; ATLANTA TWR ISSUED A CLRNC TO HOLD SHORT OF RWY 9L; WHICH WAS BEING USED FOR TKOFS. THE COPLT ACKNOWLEDGED THE HOLD SHORT CLRNC. APCHING THE HOLD SHORT POINT I THOUGHT THAT I HEARD THE TWR ISSUE A CLRNC FOR OUR FLT TO CROSS RWY 9L. AS I HAD NOT YET COME TO A FULL STOP IT WAS EASY FOR ME TO CONTINUE MY ROLL. AS I ENTERED ONTO RWY 9L I HEARD THE TWR SAY 'WHOA; FLT XYZ.' THE COPLT THEN TOLD ME THAT I DID NOT HAVE CLRNC TO CROSS. IT WAS TOO LATE AT THAT POINT TO HOLD SHORT SO I EXITED RWY 9L ON THE OTHER SIDE. IT WAS OBVIOUS THAT I WAS IN VIOLATION OF RWY 9L GND SPACE. A COMMUTER ACFT HAD STARTED HIS ROLL ON THE FAR END OF RWY 9L AND HAD TO ABORT HIS TKOF. THIS WAS POTENTIALLY A VERY SERIOUS SIT. I WILL BE THE FIRST TO ADMIT THAT IT WAS REALLY A STUPID THING TO DO FOR SOMEONE WHO HAS BEEN FLYING FOR SO LONG. POOR JUDGEMENT AND DECISION MAKING -- MOST DEFINITELY A BREAKDOWN IN CREW COM. I SHOULD HAVE CONFIRMED THE RWY XING WITH MY COPLT. I HAVE LEARNED A VALUABLE LESSON -- CONSTANT VIGILANCE. THANK GOODNESS IT WASN'T MORE SERIOUS.