DC9 FLC GIVEN TURNOFF CLRNC WHILE ON LNDG ROLL AT DCA.

Narrative

AFTER A NORMAL APCH AND LNDG ON RWY 36 AT WASHINGTON NATIONAL (DCA); THE TWR RADIOED SOME INSTRUCTIONS TO US ON LNDG ROLLOUT AND SAID THERE WAS AN ACFT ON SHORT FINAL. OF THE INSTRUCTIONS READ TO THE CAPT AND MYSELF WAS; IF UNABLE TO CLR THE RWY AT TXWY N; THEN (SOMETHING ABOUT THE INTXN OF RWYS 33 AND 36). DUE TO THE LOUD NOISE OF THE ENG REVERSE THRUST AND THE CALLOUTS OF MYSELF; THE PNF; THE END OF THE XMISSION WAS NOT CLRLY HEARD. THINKING THAT ANOTHER ACFT WAS ON SHORT FINAL; WE BOTH ASSUMED THAT WE WERE TOLD TO CLR RWY 36 AT THE INTXN OF RWY 33. AS IT TURNED OUT; THERE WAS AN ACFT ON FINAL FOR RWY 33; AND WE WERE TOLD TO HURRY THROUGH THE INTXN IF UNABLE TO CLR THE RWY ON TXWY N. TWR THEN SAID TO EXPEDITE OFF OF RWY 33; WHICH WE DID ONTO TXWY K; WELL BEFORE THERE WAS ANY CONFLICT WITH THE LNDG ACFT. THE PROB COULD HAVE BEEN AVOIDED IF WE WERE TOLD PRIOR TO LNDG TO PROCEED THROUGH THE INTXN DUE TO LNDG TFC ON RWY 33; INSTEAD OF GIVING INSTRUCTIONS DURING A VERY BUSY AND NOISY PHASE OF FLT.