AN ACR FAILED TO MAKE THE PRESCRIBED RWY TURNOFF.

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WE WERE ROLLING OUT ON RWY 18 AT DCA. ABOUT 1/2 WAY DOWN THE RWY; THE TWR CTLR BEGAN GIVING US INSTRUCTIONS TO GET OFF THE RWY. HE GAVE US SEVERAL 'CHOICES' WHILE HE WAS ALSO GIVING OTHER ACFT LNDG INSTRUCTIONS. THIS IS A VERY INTENSE TIME DURING THE ROLLOUT. WE WERE IN THE PROCESS OF BRAKING THE ACFT TO A STOP; BRINGING THE ENGS OUT OF REVERSE; AND CHANGING CTL OF THE AIRPLANE FROM THE FO BACK TO THE CAPT. IT WAS DUSK AND HAZY OUT AND IT WAS DIFFICULT TO SEE THE TAXI/RWY SIGNS WHERE RWYS 18 AND 21 CROSS. FROM THE CTLR'S DIRECTIONS; IT SOUNDED LIKE SOMEONE WAS LNDG CLOSE BEHIND US; SO ONE OF HIS OPTIONS WAS FOR US TO TURN AT TXWY D. WE HAD TO GET ON THE BRAKES TO MAKE THE TXWY. AS WE WERE CLRING; THE TWR CTLR SAID THAT WE WEREN'T WHERE HE WANTED US TO BE. THE CAPT CALLED THE CTLR ON THE PHONE AFTER WE WERE AT THE GATE. HE EXPLAINED HOW HIS INSTRUCTIONS AMOUNTED TO A 'LAND AND HOLD SHORT' INSTRUCTION WHILE WE WERE ON LNDG ROLL. IT IS DIFFICULT TO PICK UP THE SIGNS WHERE RWYS 18 AND 21 CROSS DURING LOW LIGHT CONDITIONS AND HAZE. SUPPLEMENTAL INFO FROM ACN 313846: WE WERE PRACTICALLY ABEAM TXWY F WHEN TWR MADE HIS CALL. BECAUSE OF THE HIGH COCKPIT WORKLOAD AT THE MOMENT; A REAL STATE OF CONFUSION ENSUED. I TOOK CTL OF THE ACFT AND I INTERPRETED THE TWR CTLR'S RAPID AND CONFUSING XMISSION TO MEAN HE WANTED ME CLR OF RWY 18 ASAP AND WANTED ME TO MAKE THE 'NEXT TURNOFF' WHICH HAPPENED TO BE RWY 21 AND TXWY D. A PLT SIMPLY CANNOT DIVERT HIS/HER ATTN AWAY FROM THIS TRANSITIONAL PHASE OF FLT IN ORDER TO CONSULT TXWY DIAGRAMS TO COMPLY WITH RAPIDLY WORDED INSTRUCTIONS.