UPON LNDG; CREW EXITED THE RWY AT THE FIRST AVAILABLE EXIT. ONCE CLR; THEY CONTINUED TO TAXI ON THE PARALLEL WITHOUT FIRST CONTACTING GND.

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UPON LNDG ON RWY 1L AT IAD; WE EXITED THE RWY AT TXWY YB; AND AS PER COMPANY PROC; I (AS FO) WAS TO CONTACT GND AFTER XING THE HOLD LINE ND THE SO WAS TO CONTACT THE COMPANY FOR GATE ASSIGNMENT. WE BOTH DID AS WE WERE TOLD TO DO AND AS I LOOKED UP FROM SELECTING THE GND FREQ I SAW THAT THE CAPT HAD CONTINUED HIS TAXI TO THE ADJOINING TXWY Y. I DID SEE OFF TO OUR L A J41 TAXIING S ON TXWY Y. BUT HE WAS AT LEAST 1000 FT AWAY AND POSED NO DANGER OF COLLISION. I FELT IT IMPORTANT TO IMMEDIATELY CONTACT GND RATHER THAN SAY SOMETHING TO THE CAPT SINCE THERE WAS NOWHERE TO GO OR ANYTHING TO DO ABOUT THE SIT. I CONTACTED GND WHO IMMEDIATELY; IN A VERY AGITATED TONE; TOLD US THAT WE HAD TAXIED ONTO AN ACTIVE TXWY AND HAD 'CUT OFF' ANOTHER ACFT (THE J41). HE ISSUED TAXI INSTRUCTIONS TO US AND TOLD US TO KEEP IN MIND WHAT WE HAD DONE. THE SCENARIO OF KEEPING THE ACFT MOVING AFTER EXITING THE RWY IS NOT UNCOMMON TO US. IN ORD WE HAVE NOTAMS THAT TELL US TO KEEP GOING WHEN EXITING EVEN IF WE CAN'T MAKE RADIO CONTACT SO AS NOT TO SLOW TFC FLOW EXITING THE RWY (WE HAD A JET LNDG RIGHT BEHIND US AND TWR THANKED US FOR THE EARLY EXIT). IN LGA YOU CAN'T TALK TO GND HALF THE TIME ON EXITING THE RWY AND IF YOU STOPPED AT THE HOLD LINE THERE WOULD BE NO EXITS FOR LNDG ACFT (THEY EXPECT YOU TO CONTINUE). THE FACT THAT THIS WAS THE END OF AN 11+ HR DAY COUPLED WITH AN XA00 MORNING SHOW AT THE ARPT MADE THIS CREW A VERY TIRED ONE. THE CAPT IS SO USED TO CONTINUING TO TAXI AFTER LNDG THAT IT WAS 'NORMAL' FOR HIM TO CONTINUE TO HELP INCREASE THE ARR RATE AT THE ARPT. IT HAS BECOME NORMAL FOR THE FO AND SO TO GO RIGHT TO THE RADIOS TO GET CLRNC WHEN WE SHOULD BE SURE THE STOP IS MADE. I MAKE NO EXCUSES FOR NOT LOOKING FOR THE STOP; I ONLY AM SUGGESTING WHY I THINK I DIDN'T.