AN MD80 ON VISUAL APCH TO DFW EXECUTED A GAR BECAUSE TWR CLRED AN ACFT TO CROSS THE RWY.

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WE WERE CLRED FOR A VISUAL APCH TO RWY 17C. ON SHORT FINAL ON THE ILS GS AT ABOUT 600 FT; I NOTICED AN ACR Y ACFT TAXIING CLOSE TO THE RWY EDGE. AT 400 FT WE STARTED A GAR BECAUSE THE ACR Y ACFT HAD JUST ENTERED THE L EDGE OF THE ACTIVE RWY. WE MIGHT HAVE GOTTEN AS LOW AS 300 FT DURING THE GAR. THE ACR Y ACFT WAS MOVING EXTREMELY SLOW WHILE XING THE ACTIVE RWY. IF WE HAD CONTINUED THE LNDG WITHOUT ALTERING OUR FLT PATH WE WOULD HAVE PASSED RIGHT OVER THE TOP IF NOT HIT THE ACR Y ACFT. THIS MISTAKE CAUSED BY THE TWR WAS INTENTIONAL. I TALKED TO THE SUPVR RIGHT AFTER I ARRIVED AT OUR GATE. HE TOLD ME AS LONG AS THEIR ALARMS DON'T GO OFF THEN WE HAD PROPER SEPARATION. HE WAS CONVINCED THEY DID NOTHING WRONG. WE HAD A CAPT FROM ANOTHER AIRLINE IN OUR JUMPSEAT AND HE AGREED WE WOULD GONE RIGHT OVER OR HIT THE ACR Y ACFT IF WE HAD CONTINUED THE LNDG. SO WHAT HAPPENED TO COMMON SENSE? WHEN THE TWR CLRED ACR Y TO CROSS THE RWY; THEY TOOK A VERY LONG TIME TO GET GOING AND THAT'S WHAT CAUSED THE GAR. WHY DIDN'T THEY SEE OUR ACFT ON VERY SHORT FINAL COMING RIGHT AT THEM? WHY DIDN'T THE TWR SEE THE ACR Y ACFT TAKING TOO MUCH TIME GETTING ACROSS THE ACTIVE? DURING THE GAR BEFORE I SWITCHED OVER THE DEP FREQ I HEARD THE ACR Y PLT ASK WHAT THE PROB WAS. THE TWR REPLIED THERE WAS PLENTY OF SEPARATION AND SHE DIDN'T KNOW WHAT THE PROB WAS. NEITHER THE TWR CTLR OR THE ACR Y PLTS HAD A CLUE OF WHAT HAPPENED.