AN ATCT LCL CTLR CLRED AN ACR FREIGHT ACFT FOR TKOF TOWARD AN APCHING ACR LDNG ACFT; FOR THE SAME RWY. THE RPTR'S CREW OF THE FREIGHT ACFT NOTICED THE LIGHTS APCHING THEIR DIRECTION OF TKOF; AND QUESTIONED THE CTLR. THE CTLR THEN INSTRUCTED THE RPTR'S ACFT TO TAXI CLR OF THE RWY; AND THE ARRIVING ACFT LANDED LESS THAN 1 MIN LATER.

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WE HAD A SIT OCCUR AT IAD. AFTER TAXIING TO RWY 30; WE CALLED TWR AND ADVISED THAT WE WERE 'APCHING RWY 30 FOR TKOF.' WE WERE CLRED INTO 'POS AND HOLD' WITH ADDITIONAL INSTRUCTIONS OF TFC TO OVERFLY THE ARPT. WE HAD BEEN WATCHING SOME ACFT LIGHTS THAT WERE IN THE DEP CORRIDOR FOR RWY 30 AND WE ASSUMED THAT THE ACFT WE SAW WAS THE TFC THAT THE TWR WAS TALKING ABOUT. SHORTLY AFTER GETTING INTO 'POS AND HOLD;' IAD TWR CLRED US TO TKOF. WE WERE SURPRISED ABOUT THE CLRNC BECAUSE THE LIGHTS IN THE DEP END HAD NOT MOVED CLR OF OUR DEP FLT PATH. WE QUESTIONED THE TWR ABOUT THE TFC. TWR MUST HAVE MISUNDERSTOOD US BECAUSE HE SAID SOMETHING ABOUT AUTOMOBILE LIGHTS AT THE END OF THE RWY. WE QUESTIONED HIM AGAIN TO CLARIFY WHAT WE SAW AND ABOUT THAT TIME THE PLT OF AN ACR ACFT CALLED 'FINAL FOR RWY 12.' THE CTLR IN THE TWR THEN REALIZED THE PROB AND INSTRUCTED US TO CLR THE RWY. WE COMPLIED WITH THE INSTRUCTIONS. THE ACR FLT LANDED WITHIN 1 MIN OF US EXITING THE RWY. HAD WE NOT SEEN THE OTHER ACFT WE WOULD HAVE TAKEN OFF AND OUR DEP PATH WOULD HAVE COLLIDED WITH INCOMING ACFT. OUR ACFT DO NOT HAVE TCASII TO PREVENT POTENTIAL COLLISIONS.