CAPT OF A DC10 MADE A GAR WHEN A DEPARTING DC9 WAS STARTING TKOF ROLL; WHILE HE WAS 200 FT HIGH OVER THE MM.

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UPON ARR AT THE DTW ARPT; APCH CTL CLRED US FOR THE ILS APCH TO RWY 21R. WE WERE FOLLOWING ANOTHER ACFT TO RWY 21R. THE CLRNC WAS; MAINTAIN 3000 FT UNTIL ESTABLISHED; 170 KTS TO THE MARKER; CLRED FOR THE ILS RWY 21R APCH. WE WERE ESTABLISHED IN THE LNDG CONFIGN OUTSIDE THE OM MAINTAINING 170 KIAS. AT THE MARKER WE REDUCED SPD TO OUR FINAL APCH SPD OF 143 KIAS. WE RPTED IN TO THE TWR AT THE MARKER. THE TWR CLRED US TO LAND AND ADVISED US THAT THERE WOULD BE ONE DEP BEFORE OUR ARR. AFTER THE ACFT AHEAD OF US LANDED; THE TWR CLRED AN ACR Y FLT INTO POS AND HOLD. THE TWR ADVISED HIM THAT TFC WAS 3 1/2 MI OUT. FROM OUR VANTAGE POINT; IT DID NOT LOOK LIKE 3 1/2 MI TO THE RWY; IT LOOKED MORE LIKE 2-2 1/2 MI. WITH THAT IN MIND; I BRIEFED THE GAR PROC IN CASE IT WAS REQUIRED. THE FLT THAT LANDED AHEAD OF US WAS JUST STARTING TO TURN OFF THE RWY WHEN THE TWR CLRED THE ACR Y ACFT FOR TKOF. IT LOOKED LIKE WE WERE ONLY 1-1 1/2 MI FROM THE RWY AT THAT POINT. THE ACR Y ACFT STARTED HIS TKOF ROLL AS THE PROCEEDING ACFT WAS CLRING THE RWY. AT THAT POINT; WE WERE READY TO DO A GAR. IT WAS BECOMING CLR THAT THERE WOULD NOT BE ENOUGH TIME FOR THE DEPARTING ACFT TO GET AIRBORNE BEFORE OUR TOUCHDOWN. I HAD MADE THE 200 FT CALLOUT (WE WERE LESS THAN 200 FT AGL) AND WAS REACHING FOR THE MIKE TO DECLARE A GAR JUST AS THE TWR TOLD US TO GAR. WE PERFORMED THE GAR PROC AND WERE VECTORED BACK FOR ANOTHER APCH. THAT LNDG TOOK PLACE WITHOUT INCIDENT. I INFORMED OUT FLT OPS DEPT OF WHAT HAPPENED. THEY IN TURN CONTACTED THE DTW TWR SUPVR. THEY WERE INFORMED THAT THE CTLR WORKING RWY 21R WAS IN TRAINING AND HAD JUST 6 HRS ON THE POS. THE SUPVR ADMITTED THAT THE GAPSHOT WAS TOO CLOSE TO SQUEEZE THE DEP IN. AFTER THE GAR; TWR DID GET US PRIORITY BACK IN TO THE LINE UP. FLT OPS ALSO EXPRESSED CONCERN THAT THE DEPARTING DC9 WAS NOT TOLD 'IMMEDIATE' OR 'EXPEDITE' IN THEIR TKOF CLRNC. TWR SUPVR STATED THAT THIS WAS AGAIN AN ERROR ON THE CTLR'S PART. IN MY MIND; IN AN EFFORT TO GET AN ACFT OUT AHEAD OF OUR ARR; PROPER ARR/DEP SEPARATION WAS SACRIFICED. THIS IS A COMMON PRACTICE AT CONGESTED ARPTS DURING RUSH HRS.