B737 TAXIING FOR DEP AT IAD IS CONFUSED WITH RTE ASSIGNMENT.

Narrative

TAXI FOR TKOF. TAXIING WBOUND FROM 'E' RAMP; OUR ACFT WAS GIVEN INSTRUCTIONS TO TAXI TO RWY 30 VIA TXWY Z AND TXWY Y11. THIS INSTRUCTION WAS VAGUE; SINCE NO OTHER TXWYS WERE MENTIONED AFTER TXWY Y11; IE; TXWY Y; TXWY Z; OR TXWY Q. THE TAXI TIME WAS SHORT AND 2 DISTRS DEVELOPED -- ONE BEING A CALL FROM THE FLT ATTENDANTS INVOLVING A BROKEN SEAT AND RELOCATING A PAX; THE SECOND INVOLVING A HAZMAT PAPERWORK CHK AGAINST OUR LOAD CLOSEOUT. TAXIING SBOUND ON TXWY Z; THE OPTICS MAKE IT APPEAR THAT TXWY Y11 ENDS/BEGINS ON THE W SIDE OF THE HOLD PAD; THUS OUR ONLY OPTION TO TURN R (W) AWAY FROM THE DIRECT PATH TO RWY 30; S ON TXWY 1. THIS PUZZLED MYSELF AND THE FO; WHO QUERIED GND CTL AS TO ROUTING AND THAT RWY FULL LENGTH WAS REQUIRED. GND CTL RESPONDED; 'TXWY Y11; FULL LENGTH; SWITCH TWR.' WITH THE MIND SET TO BOX PATTERN MY WAY TO RWY 30; I TOOK THE R ONTO TXWY Y11 (NWBOUND) POSSIBLY VIOLATING THE RWY 1L APCH AREA (DULLES WAS LNDG RWY 1L). NOW ON TWR FREQ; TWR ROUTED THE ACFT S ON TXWY Y; THEN SE ON TXWY Q (BRIEF HOLD SHORT TXWY Z); S ON TXWY Q. CONTRIBUTING FACTORS 1) VAGUE INSTRUCTIONS FROM GND CTL; IE; NO MENTION OF L OR R ON TXWY Y11 OR TAXI RTE AFTER TXWY Y11. 2) DISTRS DURING A SHORT TAXI; NOT ALLOWING PROPER REVIEW OF THE COMMERCIAL CHART PAGE FOR TAXI RTE ANALYSIS. 3) INDECISIVENESS ON MY PART. I DID ADEQUATELY RESOLVE MY DOUBTS ABOUT THE ROUTING; EVEN AFTER THE QUERY TO GND CTL BY THE FO. I SHOULD HAVE REQUESTED MORE DEFINITIVE INSTRUCTIONS BEFORE PROCEEDING AND NOT ALLOWED THE KNOWLEDGE THAT ACFT WERE BEHIND ME (STOPPING THE CHAIN) TO PRESSURE ME INTO A HASTY DECISION WHICH TOOK ME AWAY FROM THE OBVIOUS ROUTING WHICH I HAD TAKEN MANY TIMES IN THE PAST. I ALSO FEEL THAT MORE COMPLETE FULL RTE TAXI INSTRUCTIONS; IE; L OR E; ALL TXWYS TO BE USED ETC; MAY HAVE AVOIDED THIS INCIDENT.