POTENTIAL CONFLICT AS SMT EXPERIENCES RWY INCURSION. UNAUTHORIZED UNCOORD RWY ENTRY.

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THE ARPT WAS VERY BUSY WITH ARRIVING ACFT AS WELL AS BANNER TOWING ACFT. UPON LNDG THE TWR CTLR INSTRUCTED ME TO 'TAKE NEXT TAXIWAY AND STAY WITH ME.' I TOOK THE NEXT TAXIWAY AND PROCEEDED TO THE RAMP. THE TWR CTLR WAS VERY BUSY WITH ALL THE TFC IN HER AIRSPACE. THE NEXT COM I HEARD DIRECTED TO MY ACFT WAS IN AN EXCITED VOICE 'SMT X; HOLD SHORT 30.' UPON RECEIVING THIS COM; I BEGAN BRAKING AND LOOKING FOR RWY 30. I IMMEDIATELY REALIZED I WAS ABOUT TO BEGIN XING RWY 30 WHICH WAS A SOD RWY AND APPLIED FULL BRAKING PRESSURE. SIMULTANEOUSLY; A BANNER PLT EXECUTED A GAR ON 30 AS HE WAS FLYING A LOW APCH TO DROP A BANNER. I BELIEVE I COULD HAVE STOPPED BEFORE ARRIVING AT RWY 30 BUT THE BANNER PLT WAS CORRECT IN EXECUTING THE FAR AS THERE WAS LESS MARGIN OF SAFETY THAN NORMAL. THE TWR THEN CAME BACK AND CLRED ME TO CROSS 30 AND SAID 'DIDN'T YOU UNDERSTAND MY INSTRUCTIONS TO HOLD SHORT?' I RESPONDED THAT I HAD NEVER ACKNOWLEDGED ANY SUCH INSTRUCTION (OTHER THAN THE LAST ONE WHICH WAS TOO LATE TO MAKE A REASONABLE STOP). LATER; ON THE TAXI TO THE RAMP; ANOTHER CTLR CAME ON AND ASKED ME THE SAME QUESTION AND MY RESPONSE WAS THE SAME. I AM CONVINCED THAT I WAS NEVER GIVEN 'HOLD SHORT' INSTRUCTIONS PRIOR TO THE EXCITED INSTRUCTIONS WHICH WERE VERY LATE. I ALSO FEEL THAT THE ACFT COULD HAVE STOPPED IN TIME EVEN WITH THE LATE INSTRUCTIONS. ON THE OTHER HAND; I SHOULD NOT HAVE PRESUMED THAT HER INSTRUCTIONS CLRED ME TO THE RAMP EVEN THOUGH THAT OFTEN HAPPENS WHEN A CTLR IS BUSY. THE FACTORS THAT INFLUENCED THIS INCIDENT; IN MY OPINION; INCLUDE: MY FIRST TIME IN ARB. A VERY BUSY ARPT WITHOUT RADAR; AND 1 CTLR. A SOD RWY WHICH ISN'T AS NOTICEABLE FROM THE TAXIWAY. BANNER AIRPLANES WORKING AND INTERSECTING RWY FROM THE ACTIVE (VERY ACTIVE; I WAS SECOND IN LINE TO LAND OF 3).