FLT CREW OF B757 MISINTERPRET INTENT OF GND CTLR REQUEST EXITING RWY 1L AT IAD. CTLR COUNSELS FLT CREW FOR DELAYING SUBSEQUENT DEP.

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ON LNDG ROLLOUT RWY 1L; WE GOT INSTRUCTIONS TO EXIT AT Y2 AND CONTACT GND ON 121.9. THE CAPT BEGAN TO TURN OFF IN THE RAIN; AND I PICKED UP THE SIGN FOR Y2 TXWY. THE SIGN SHOWS A 120 DEG TURN OFF THE RWY. I INDICATED THIS TO THE CAPT AND HE AGREED; AND WE BOTH WENT BACK TO LOOKING OUTSIDE AND EXITING THE RWY. AFTER WE CLRED RWY 1L; I SWITCHED TO GND AND BEGAN RUNNING THE AFTER LNDG CHKLIST. THE CAPT SLOWED THE ACFT TO MAKE THE TURN AND PROCEEDED DOWN TXWY Y2. GND WAS BUSY AT THE TIME AND WE GOT TO THE 'Y' TXWY BEFORE SHE TALKED TO US. SHE TOLD US WE MESSED UP THE TAXI INSTRUCTIONS AND CAUSED A TFC DELAY ON THE TKOF RWY. SHE ALSO SAID SHE DID NOT APPRECIATE US IGNORING THE TAXI INSTRUCTIONS. I TOLD HER WE EXITED ON Y2 AS INSTRUCTED AND SHE SAID WE NEEDED TO CHECK THE ARPT MAP AND IT SHOWED US ON TXWY Y3. WE WERE ALSO TOLD THAT THE TWR CTLR SAID AT TXWY Y2 HIGH SPEED. WE BOTH DID NOT HEAR THE WORD HIGH SPEED. THE CAPT CALLED THE TWR AND TALKED TO THE SUPERVISOR. HE EXPLAINED TO THE CAPT THAT THIS HAPPENS A LOT AND THE TWR CTLRS ARE UPSET BECAUSE THEY THINK THE AIRCREWS ARE JUST IGNORING THE TWR'S INSTRUCTIONS. THE CAPT TOLD HIM AIRCREWS ARE NOT ACCUSTOMED TO JUST IGNORING INSTRUCTIONS AND THAT THE REAL PROB IS WHAT THE SIGNS ARE TELLING US GETTING OFF THE RWY. HE ALSO TOLD HIM THAT AIRCREWS ARE NOT LOOKING AT THEIR MAPS EXITING THE RWY (CRITICAL PHASE OF FLT);WE RELY ON THE SIGNS AND USE OUR MAPS ONCE WE ARE SAFELY AT TAXI SPEED. WE FILED A COMPANY RPT; AND BELIEVE THAT THE SIGNS HAVE NEED TO BE LOOKED AT.