CRJ200 FLT CREW IS ISSUED TKOF CLEARANCE AT DCA WITH WITH PREVIOUS LANDING ACFT STILL ON THE RWY.

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DCA WAS DEPARTING RWY 19 AND USING THE RIVER VISUAL 19 FOR ARRS. AN ACR Y A320 LANDED AND WE WERE CLRED INTO POS AND HOLD ON RWY 19. I WITNESSED AN ERJ ON APPROX A 3 MI FINAL. THE TWR THEN CLRED US FOR TKOF; HOWEVER; THERE WAS A LONG DELAY BEFORE HE SAID 'TKOF.' THE A320 WAS STILL CLRING THE RWY AROUND TXWY F OR SO. HE WAS FACING THE TXWY BUT THE ENTIRE ACFT WAS STILL ON THE RWY. THE FO RESPONDED BY SAYING; 'WE WILL WAIT UNTIL THE ACFT IS CLR OF THE RWY; OK; ACR Z; CLRED FOR TKOF.' HE SAID IT ALL IN 1 XMISSION AND THE A320 WAS NOW ALMOST ALL THE WAY CLR. WE THEN STARTED THE TKOF RUN. AT ABOUT 60 KTS; I SAID; 'THRUST SET;' AT THE SAME TIME I HEARD OVER THE TWR FREQ; 'CANCEL TKOF CLRNC AND CLR THE RWY IMMEDIATELY.' I DID NOT HEAR THE CALL SIGN. IT TOOK A SECOND TO REGISTER HE WAS TALKING TO US SINCE THEY WERE ALSO DEPARTING RWY 15. MY INITIAL THOUGHT WAS THAT HE WANTS US TO CLR THE RWY SO THE ERJ CAN LAND. I KNEW THAT WITH THE PLANE ACCELERATING AND THE ENGS AT FULL THRUST; THE EARLIEST I COULD CLR THE RWY WOULD BE THE N TXWY WHICH I'M SURE WOULD HAVE CAUSED A GAR FOR THE ERJ. HOWEVER; I ALSO WASN'T SURE IF THERE WAS ANOTHER REASON TO CANCEL THE TKOF CLRNC; SUCH AS A SECURITY REASON; ETC. IN WHICH CASE; CLRING THE RWY HALF WAY DOWN WOULD BE FINE. I THEN XMITTED TO TWR; 'DO YOU WANT ACR Z TO ABORT OR CONTINUE?' I WANTED TO USE AS FEW WORDS AS POSSIBLE SINCE WE WERE NOW APCHING 80 KTS. THE TWR RESPONDED; 'ACR Z CLRED FOR AN IMMEDIATE TKOF.' AT APPROX 400 FT AGL THE TWR SAID WE HAVE A PHONE NUMBER FOR YOU TO COPY. I TOLD THEM I WOULD GET BACK TO THEM IN A MINUTE. AFTER 1500 FT AGL; I THEN SAID I WAS READY TO COPY. WHEN WE ARRIVED IN BOS; I CALLED THE TWR SUPVR IN DCA. HE WAS VERY CORDIAL. HE EXPLAINED THAT IT WAS PRETTY TOUGH TRYING TO GET ALL THE ARRS AND DEPS IN AND OUT OF DCA AND THAT THEY HAVE TO RUN THINGS PRETTY TIGHT. HE SAID THAT WHEN THE FO INITIALLY TURNED DOWN THE TKOF CLRNC; THEY THOUGHT SOMETHING MIGHT BE WRONG WITH US; SUCH AS MECHANICAL OR SECURITY. I THEN EXPLAINED THAT FROM OUR VIEW THE ENTIRE A320 WAS STILL ON THE RWY. I THEN ASKED HIM WHAT THE REGS FOR ATC REQUIRE BEFORE ISSUING A TKOF CLRNC WITH AN ACFT CLRING THE RWY. HE SAID THAT AS LONG AS THE ACFT IS FACING THE EXIT TXWY AND HAS UNIMPEDED ACCESS TO CLR THE RWY AND IS MOVING; THAT THEY CAN ISSUE THE CLRNC. HE THEN SAID IT WAS UP TO THE PLTS WHEN THEY START THE TKOF ROLL. I ALSO EXPLAINED TO HIM THAT I DIDN'T HEAR THE CALL SIGN ON THE 'CANCEL TKOF' BECAUSE OF OUR CALLS IN THE COCKPIT. HE SAID EVERYTHING WORKED OUT FINE; THE ERJ LANDED; AND THAT THERE WAS NOTHING FROM HIS END. I THINK THE SIT THAT CAUSED THIS STARTS IN OUR HUB WHERE WE HAVE BEEN TOLD THAT THEY WANT US TO TKOF IMMEDIATELY WHEN GIVEN A TKOF CLRNC. WE INSTINCTIVELY DON'T WANT TO ACCEPT A TKOF CLRNC UNLESS WE CAN IMMEDIATELY COMPLY WITH IT.