WDB TAXIING HAS GND CONFLICT WITH WDB EXITING RWY.

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DURING PUSHBACK; FOG ROLLED INTO PHILADELPHIA INTL. VISIBILITY DROPPED FROM 2 1/2 MI DOWN TO RVR 0-600-600. WE DISCUSSED OUR TAXI RTE WHICH INCLUDED THE FACT THAT THEY (ATC) HAD JUST CHANGED RWYS FROM RWY 27L TO RWY 9R. WE WERE CLRED 'RAMP TO K3 CROSS RWY 9L (CLOSED) AND RPT XING; PROCEED M TO S TO YANKEE; HOLD SHORT AND CALL TWR AT YANKEE AND ADVISE TWR. WE COMPLIED WITH TAXI AND CALLED XING RWY 9L; WE ALSO HAD OUR STROBES AND ALL OUTSIDE LIGHTS ON DUE TO THE POOR TAXI VISIBILITY. DURING TAXI; A GND VEHICLE CALLED A COUPLE OF TIMES FOR SURFACE RPTS. WE UPDATED OUR POS AS S2-GND ASKED OUR POS SHORTLY AFTER THAT RPT AND I ADVISED' ON SIERRA XING UNIFORM NOW.' VISIBILITY NOW RPTED AS RVR 600-800-600. GND SAID PROCEED TO YANKEE-CROSS YANKEE AND CALL TWR XING AND ADVISE TWR OF YOUR POS. OUR NOSE HAD JUST TOUCHED S4 WHEN WE SAW LIGHTS COME OUT OF THE FOG. THE FIRST INSTANT WE THOUGHT IT WAS THE GND VEHICLE WE HEARD ON THE FREQ; BUT THE NEXT INSTANT WE NOTICED A 767 ATTACHED TO THE LIGHTS TOWARD US ON THE HIGH SPD EXIT AT S4. THIS NEXT EFFORT HAPPENED AT 1 TIME. THE CAPT APPLIED LARGE FORWARD THRUST AND STEERED TO THE FAR R OF THE TXWY; I CALLED OUT ON THE FREQ. 'TELL ACR TO STOP;' TELL ACR TO STOP;' IN A CLR COMMAND INSTRUCTION; AND OUR SO WAS CALLING 'WATCH IT WATCH IT.' THE 767 HIT HIS BRAKES AND TURNED FOR HIS R SIDE OF S4 AND STOPPED. WE TURNED BACK TO CTR LINE AND ADVISED GND. NO DAMAGE OR INJURY. GREAT CLR ON OUR CREW. WE WERE ALWAYS AWARE OF OUR POS; AND HAD OUR HEADS UP DURING THE TAXI. WE CONSTANTLY NOTED OUR POS AND KNEW OF INTXNS. WE HAD NO HESITATION TO GET OUT OF THE WAY OF THE 767. ALL OF US CONTRIBUTED TO AVOIDING THE 767. OUR BRIEF ON THE RWY CHANGE; AND SITUATIONAL AWARENESS HAD US PREPARED FOR ANYTHING; AND OUR QUICK RESPONSE TO THE 767 PREVENTED METAL FROM BENDING! WE WERE TOLD LATER; THE 767 WAS TO EXIT AT S1 AFTER ANY AUTOMATIC LNDG. ATC SHOULD ADVISE LNDG TFC THAT OPPOSITE TXWY IS HOT; ALSO HAVE RWY EXIT PLAN BRIEFED PRIOR TO LNDG.