WHILE ASSUMING HE WAS FOLLOWING TAXI INSTRUCTIONS; CAPT INADVERTENTLY TRANSGRESSED ONTO AN ACTIVE RWY WHILE ANOTHER MD80 WAS ON ITS TKOF ROTATION. CAPT THEN TAXIED ACROSS THE ACTIVE RWY TO AVOID DEPARTING ACFT.

Narrative

AFTER COPLT'S LNDG AT EWR; RWY 4R I TOOK/RECEIVED CTL OF THE ACFT JUST PRIOR TO HIGH-SPD TXWY L. ON TXWY L WE WERE ISSUED CLRNC 'TURN L ON TXWY Y AND L ON TXWY P WHICH THE COPLT READ BACK. I CONTINUED ON TXWY L WHILE CONTINUING TO SLOW THE ACFT AND CHKED FOR TXWY MARKERS. I SAW TXWY Y; MAD THE TURN L ONTO TXWY Y AND THEN LOOKED FOR TXWY P AND BELIEVED IT TO BE ON THE OTHER SIDE OF RWY 4L. APCHING RWY 4L CTRLINE TWR COMMANDED 'STOP' AND THE COPLT QUESTIONED 'WHERE ARE YOU GOING?' AT ABOUT THAT SAME TIME I LOOKED L TO SEE AN MD80 IN TKOF ROTATION. WITH THAT ACFT RAPIDLY APCHING AND KNOWING THAT I WOULD NOT BE ABLE TO STOP OUT OF HARM'S WAY I CONTINUED TO CROSS RWY 4L. ONLY UPON REACHING THE OTHER SIDE OF RWY 4L DID I REALIZE THAT TXWY P WAS THE PARALLEL TXWY BTWN RWYS 4R AND 4L. I DID PHONE AND TALK TO A TWR SUPVR WHO INDICATED THAT THIS INCIDENT WOULD BE CONSIDERED A 'PLTDEV' AND BE SENT ON FOR FURTHER REVIEW/ACTION. CONTRIBUTING TO MY NEAR CATASTROPHIC ERROR WAS THE CLOSE POS OF TXWY P AFTER TURNING ONTO TXWY Y FROM TXWY L. WITH THE RECEIPT OF TAXI INSTRUCTIONS ON TXWY L WITH THE ACFT STILL DECELERATING THERE WAS LITTLE TIME TO DIGEST THE CTLR'S INTENTION FOR US. I WAS QUICKLY ABEAM; IF NOT PAST TXWY P ON TXWY Y. MY INCORRECT ASSUMPTION THAT TXWY P WAS ON THE OTHER SIDE OF RWY 4L COUPLED WITH NOT RECEIVING 'CLRNC TO CROSS RWY 4L' ARE THE KEY ERRORS IN THIS SIT. I DO BELIEVE THE CTLR COULD HAVE HELPED US AVOID THIS WITH A BIT MORE TAXI INFO AND A DEFINITIVE 'HOLD SHORT OF RWY 4L; ACFT ON TKOF ROLL!' IE; 'ACR AFTER TXWY Y MAKE THE QUICK HARD L ONTO TXWY P BTWN THE RWYS; ETC.' MY BOTTOM LINE IS: WITHOUT EQUIVOCATION I WAS WRONG TO CONTINUE ON TXWY Y UNTIL I WAS 100 PERCENT SURE WHERE TXWY P WAS AND I SHOULD NOT HAVE BEEN ON THAT RWY 4L WITHOUT XING INSTRUCTIONS. I WILL NOT CROSS A RWY UNTIL POSITIVE THAT CLRNC HAS BEEN RECEIVED AND I WILL VISUALLY CHK (CAREFULLY EXECUTED) THAT THE RWY IS SAFE TO CROSS.