TWR INTERVENED TO STOP MLG SHORT OF RWY DURING TAXI.

Narrative

ACFT WAS TAXIING AS INSTRUCTED NE ON RWY 6 TO HOLD SHORT OF RWY 12R. TFC CONDITION AT THE ARPT WAS BUSY WITH SOME PERIODS OF CONSTANT RADIO USE ON GND CTL FREQ WHILE BRAKING THE ACFT AND JUST PRIOR TO REACHING A COMPLETE STOP ABOUT 30 FT SHORT OF RWY 12R GND CTL ISSUED EMPHATIC INSTRUCTIONS TO STOP SHORT OF RWY 12R. THE ACFT WAS STOPPED WELL CLR OF RWY 12R. IMMEDIATELY AFTERWARD; GND CTL ISSUED INSTRUCTIONS TO CROSS RWY 12R WHILE ADDING WORDS THAT INDICATED OUR POS RELATIVE TO THE ACTIVE (RWY 12R) CAUSED A CONFLICT. WE CROSSED RWY 12R AND CONTINUED THE FLT. WE HAD HELD SHORT OF THE RWY AT A LOCATION WHICH I THOUGHT WOULD BE IN LINE WITH OTHER HOLD SHORT POS FOR THAT RWY AND WHICH ALLOWED CLRNC OF THE INTERSECTING TXWY A BEHIND THE ACFT. THERE WAS NOT A HOLD SHORT MARKING ON RWY 6 FOR RWY 12R. THE ARPT MARKING SIGNS FOR RWY 12R/30L WERE SET BACK FROM RWY 12R NEAR TXWY A AND DID NOT APPEAR TO ESTABLISH A HOLD SHORT POS. THERE ARE OTHER RWY HOLD SHORT MARKINGS ON RWY 6; BUT NONE ON RWY 6 SW OF RWY 12R. THE POS OF THE CTL TWR WAS DIRECTLY IN LINE AND BEHIND OUR STOPPED POS; SO GND CTL MAY NOT HAVE BEEN ABLE TO JUDGE OUR DISTANCE FROM THE RWY. RWY 6 IS OFTEN USED AS A TXWY IT SHOULD BE MARKED WITH A HOLD SHORT LINE FOR RWY 12R.