XING TFC -- ACR CREW FOUND THAT THEY HAD TO HURRY ACROSS THE RWY AS APCHING ACR IS TOO CLOSE.

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SLC TWR ON 119.05 ISSUED US CLRNC TO CROSS RWY 34 AND CONTINUE TAXI TO RWY 35 AND MONITOR TWR ON 118.3 AFTER XING RWY 34. THERE WERE AIRPLANES ON FINAL TO RWY 34 THE CLOSEST OF WHICH WE JUDGED TO BE 4.5-5 MI OUT. BECAUSE IT WAS NIGHT TIME WITH ONLY LNDG LIGHTS VISIBLE ON THE TFC IT WAS IMPOSSIBLE TO ACCURATELY JUDGE DISTANCE AND CLOSURE RATE. WE WERE LARGELY DEPENDENT ON THE TWR'S INFO. TFC TURNED OUT TO BE CLOSER AND BY THEN COMMITTED. I HAD TO HURRY TO MAKE THE XING. WE DID GET ACROSS AND CLR WITH NO COMMENT FROM TWR ON 119.05 WHICH WE WERE STILL ON AS INSTRUCTED OR FROM THE TFC BUT SEPARATION WAS MUCH TOO CLOSE FOR MY COMFORT. CLRNC TO CROSS SHOULD NOT HAVE BEEN GIVEN OR AT LEAST GIVEN ONLY WITH A COMMENT ABOUT DISTANCE AND INSTRUCTION TO EXPEDITE. I WOULD HAVE HELD SHORT IF I HAD RECEIVED ACCURATE INFO. SUPPLEMENTAL INFO FROM ACN 298385: WE WERE HOLDING SHORT IN NIGHT VISUAL CONDITIONS OF RWY 34 -- AWAITING LNDG TFC. AFTER TFC; GIVEN CLRNC TO CROSS RWY 34 WITH SECOND ACFT ON FINAL. IT WAS DIFFICULT TO DETERMINE THE DISTANCE OF THE ACFT ON FINAL; BUT APPEARED TO BE APPROX 4.5 NM VIA TCASII. AS WE CROSSED THE RWY WE DETERMINED THE ACFT WAS CLOSER WITH A HIGH RATE OF CLOSURE. WE EXPEDITED OUR XING AND ESTIMATE WE WERE CLR OF RWY 34 BY 100 FT AS ACFT (A B727) CROSSED THE THRESHOLD.