B737-900 FLT CREW QUERY THEMSELVES ABOUT WHAT THEY SHOULD HAVE DONE DURING A TIGHT SPACING EVENT WHILE LNDG AT ORD.

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UPON CHKING IN WITH ORD TWR ON THE ILS RWY 4R; WE WERE TOLD WE WERE #3 FOR RWY 4R; WITH THE FIRST TFC TOUCHING DOWN AND SECOND TFC 2.5 MI AHEAD. WE BEGAN TO SLOW ABOUT 2 MI PRIOR TO FAF (TO FINAL APCH SPD) IN ORDER TO MAINTAIN THE 2.5 MI SEPARATION FROM TFC AHEAD. AS TFC AHEAD TOUCHED DOWN; ORD TWR INSTRUCTED THEM TO ROLL TO THE END (RWY 4R LENGTH 8071 FT) AND WAS TOLD TO EXPEDITE; BECAUSE TFC WAS 2 MI BEHIND. ORD TWR THEN CLRED US TO LAND RWY 4R. PASSING THROUGH 500 FT; IT WAS APPARENT THAT TFC WAS STILL ON THE RWY; BUT NEARING THE END. I ASKED THE CAPT TO LET ME KNOW WHEN THE TFC CLRED AND SHIFTED MY FOCUS TO THE INSTS AND TOUCHDOWN ZONE FOR LNDG. PASSING THROUGH 130 FT AND APCHING A GAR DECISION. ORD TWR ONCE AGAIN CAME ON THE RADIO AND SAID THAT TFC WAS CLRING THE RWY; AND AGAIN CLRED US TO LAND. WE CONTINUED FOR A NORMAL LNDG AND ROLLOUT. ON TAXI-IN; I COMMENTED TO THE CAPT HOW ODD IT WAS THE TWR CLRED US TO LAND A SECOND TIME; WHEN THERE WAS STILL A QUESTION IN OUR MIND ABOUT THE TFC AHEAD CLRING THE RWY. THE CAPT COMMENTED THAT THE CLRNC WAS MARGINAL; BECAUSE THE TFC HAD STILL NOT CLRED AS WE CROSSED THE THRESHOLD AND HAD JUST PARTIALLY CLRED BY TOUCHDOWN. GRANTED; THE TFC WAS 7000 FT DOWN THE RWY FROM US; BUT THIS IS STILL DISTURBING. WE SHOULD HAVE INITIATED A GAR ABOUT THE TIME TWR CAME ON THE SECOND TIME; TELLING US THE TFC WAS CLRING; AND AGAIN CLRING US TO LAND. I THINK THE SPACING WAS TOO TIGHT. WE WERE IN A 737-900 AT MAX LNDG WT; 160 KTS. TFC AHEAD WAS A 737-200 WITH A 20 KTS SLOWER APCH SPD. THE TWR ALSO SEEMED TO WANT TO AVOID A GAR WITH THE SECOND CLRNC. WE SHOULD HAVE DISREGARDED AND INITIATED A GAR. IT'S OUR DECISION.