MLG ACFT WITH TAXI CLRNC TO A RWY WAS GIVEN A SUDDEN STOP WHEN ABOUT TO CROSS ANOTHER ACTIVE RWY JUST AS AN ACFT WAS TAKING OFF.

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AFTER CALLING FOR TAXI CLRNC; THE CTLR GAVE US DIRECTIONS FOR EXITING RWY 1L 'MIKE ECHO KILO TO THE RAMP.' THE FO AND I LOOKED AT EACH OTHER AND SHE (THE FO) RESTATED TAXI FROM THE RAMP WITH ATIS XYZ...WE'LL NEED RWY 1L DUE TO WT. (THEY WERE DEPARTING RWY 7R LNDG RWY 1L.) THE CTLR APOLOGIZED FOR THE CONFUSION. HE STATED 'ACR; TAXI TO RWY 1L VIA B.' NO HOLD SHORT INSTRUCTIONS WERE GIVEN. IT WAS THE FO'S LEG AND WE WERE DISCUSSING THE PREDEP BRIEF PER COMPANY POLICY. AT THIS POINT; THE TAXI CHK WAS COMPLETED; WE MADE THE L TURN TO CROSS (OR PREPARE TO CROSS) RWY 7R. THE FO WAS FINISHED UP WITH THE PREDEP BRIEF; WHEN OVER THE RADIO CAME; 'ACR X HOLD SHORT!' I SLAMMED ON THE BRAKES; THE NOSE AND THE FRONT WHEEL WAS JUST OVER THE HOLD SHORT LINE. BOTH THE FO AND I; THE CAPT; LOOKED TO OUR R AND SAW AN ACR Y DC9 ON THE TKOF ROLL. EVEN AT THIS POINT I COULD NOT BELIEVE WHAT I SAW. THE DC9 CROSSED IN FRONT OF US ABOUT 4-5 SECONDS LATER AT ROTATION. WE WERE SUBSEQUENTLY CLRED ACROSS RWY 7R AND INSTRUCTED TO MONITOR THE TWR. WE CROSSED RWY 7R AND HELD SHORT OF RWY 1L ON TWR FREQ WITH NOTHING ELSE SAID. FACTORS OF THIS INCIDENT WHICH CONTRIBUTED TO THE PROB: CONFUSION BY THE GND CTLR AS TO WHICH ACFT WE WERE; AS EVIDENCED BY THE INITIAL CALLUP AND THE INITIAL TAXI INSTRUCTIONS FOR AN ACFT LEAVING THE RWY. THE CTLR HAD IT IN HIS HEAD TO CLR US TO RWY 7R WHILE HONORING OUR REQUEST AND CLRNC TO TAXI TO RWY 1L. THIS WAS VERIFIED BY THE LACK OF THE HOLD SHORT INSTRUCTIONS NORMALLY ISSUED WITH INTERSECTING RWYS USED ON LNDG ONE RWY AND DEPARTING ON ANOTHER RWY. STOP BAR LIGHTS SHOULD BE USED AT THE INTXN OF TXWY R AND RWY 7R PRIOR TO XING. I'M GLAD I ALWAYS TAXI SLOW. HAD I BEEN FASTER; THE END RESULT COULD HAVE BEEN FAR MORE TRAGIC. WHEN I BRAKED; NO ONE COMMENTED ON THE STOP -- NOT EVEN THE FLT ATTENDANTS. TAXIING FAST WITH FLT ATTENDANTS DOING THEIR DEMONSTRATIONS COULD BE A HAZARD FOR THEM; REGARDLESS OF WHY YOU WOULD HAVE TO BRAKE HARD ON A TAXI OUT OR IN TO THE GATE. MILWAUKEE IS TELLING ACFT WHEN TO GO TO THE TWR FREQ FROM GND FREQ. IF YOU ARE TAXIING ACROSS AN ACTIVE RWY; THE ACFT SHOULD BE ON TWR FREQ; NOT GND. HAD WE BEEN ON TWR FOR THE CROSS; WE WOULD HAVE HEARD THE DC9 ON THE TKOF (CLRED FOR TKOF) AND BEEN ABLE TO STOP IN AMPLE TIME AND AVOIDED THE WHOLE INCIDENT.