MLG LNDG IAD DOES NOT COMPLY WITH ATC DIRECTION; TURNED OFF RWY AT REVERSE HIGH SPEED; RESULTING IN REQUIRED GAR ON NEXT ACFT LNDG.

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FLT TO IAD. WX CONDITIONS ON ARR: CALM WINDS; 10 MI VISIBILITY; SCATTERED SKIES. WE WERE CLRED FOR THE VISUAL APCH TO RWY 19C AT IAD. WE BRIEFED THE APCH AND LNDG TO VACATE AT TXWY Y5 (HI SPD) TO MINIMIZE TIME ON THE RWY AS INDICATED IN THE ATIS. I USED MAX REVERSE THRUST AND LIGHT BRAKING. THE ACFT WAS SLOWED SUFFICIENTLY TO BE ABLE TO MAKE THE REVERSE HI SPD TXWY Y4. TWR HAD ADVISED ON ROLLOUT TO TAKE THE NEXT HI SPD AVAILABLE (Y5). UPON EXITING IN AN EFFORT TO CLR THE HI SPD FOR THE NEXT LNDG TFC; WE REQUESTED TO TAKE TXWY Y4. THE CTLR ADVISED TO EXPEDITE AND INSTRUCTED US WE WERE TO TAKE TXWY Y5 NOT TXWY Y4. WE WERE UNABLE TO REACH RWY CTRLINE IN TIME AND THE NEXT LNDG TFC WAS INSTRUCTED TO GO AROUND BY TWR. BY TAKING THE REVERSE HI SPD (TXWY Y4) WE CREATED A SEPARATION PROB FOR THE CTLR. HAD I MAINTAINED A HIGHER SPD ON THE ROLLOUT AND EXPEDITED OFF TXWY Y5; THE NEXT LNDG TFC COULD HAVE LANDED; THOUGH THE TIMING WAS VERY CLOSE. I SHOULD HAVE REQUESTED TO EXIT ON THE REVERSE HI SPD TXWY Y4 WITH THE TWR CTLR PRIOR TO LNDG TO PREPARE THEM TO HAVE THE NECESSARY SEPARATION FROM THE FOLLOWING TFC. IN THIS SIT; THEY WOULD HAVE DECLINED OUR REQUEST AND TOLD US TO EXPEDITE ON TXWY Y5 DUE TO CLOSE SEPARATION WITH FOLLOWING TFC. IN THE FUTURE; I WILL ALWAYS EXPEDITE THROUGH THE HI SPD TXWY AND NOT ANTICIPATE GETTING A TAXI CLRNC ON THE REVERSE HI SPD. IN THE PAST WE HAVE BEEN GIVEN CLRNC FOR THE REVERSE HI SPD WITHOUT CONFLICT. THIS; HOWEVER; IS NOT THE NORMAL PROC AND BY ANTICIPATING AND NOT COMMUNICATING IN ADVANCE I CREATED THE SEPARATION PROB WITH THE LNDG TFC.