AN A320 CREW TAXIES THE WRONG WAY ON THE WAY TO RWY 30 AT IAD. THE PIC FOLLOWED A TURBOPROP AND WENT ONTO A TXWY OTHER THAN CLRED.

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CLRED BY DULLES GND FROM POS SHORT OF TXWY Z ON TXWY E FROM S RAMP TO TAXI TO RWY 30 VIA TXWY Z/Q. UPON REACHING THE SECONDARY DEICING SITE ON TXWY Z; CONTINUED TO RWY 30 VIA TXWY ON L SIDE. GND CTL QUERIED WHAT ACFT WAS TAXIING L ON Y9 TO RWY 30. REPLIED THAT WE WERE; ACR XYZ. (OUR DEST WAS MEX.) GND INFORMED US THAT WE WERE NOT ON TXWY Q AND Y9 WAS NOTAMED ON ATIS NOT FOR ACFT WITH GREATER THAN 97 FT WING SPAN. OUR A320 WING SPAN IS 110 FT. WE CONTINUED TO END OF RWY AND WAS CLRED FOR TKOF SHORTLY LATER. AS WE WERE APCHING THE DEICING AREA; A B757 WAS ON THE TXWY TO THE R WHICH IS TXWY Z AND WAS WAITING FOR FINAL WTS. A TURBOPROP TAXIED TO PASS TO THE L TO GET AROUND THE B757. WE FOLLOWED THAT DIRECTION ALSO AS THE IAD ARPT DIAGRAM APPEARS TO SHOW THE L SIDE AS TXWY Q. THE SMALL SECTION THAT IS ACTUALLY Y9 IS NOT CLEARLY DELINEATED. AFTER ENTERING THE RWY 30 POS AND HOLDING; THE Y9 DESIGNATION ON THE TXWY COULD BE SEEN MUCH BETTER. NORMAL GND CLRNCS AT IAD FOR RWY 30 IS EITHER VIA TXWY Y TO RWY 30 OR TXWY Z TO RWY 30 AND TFC FLOWS TO THE L WITH NO DIRECTION FROM GND TO DO SO. ALSO; CLARIFYING THE ARPT DIAGRAM WOULD HELP GREATLY.