TXWY EXCURSION OF A B727 PIC AS HE TAXIED TOO FAST ROUNDING THE TURN ON A SLIPPERY TXWY.

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THE RAIN STOPPED BUT THE CONDITIONS ON THE RWY WERE RPTED AS POOR TO NIL FOR BRAKING ACTION. WE SAT IN THE ACFT FOR 2 HRS BEFORE WE DECIDED TO GO. CONDITIONS HAD IMPROVED BRAKING ACTIONS WERE RPTED AS FAIR TO GOOD ON TXWYS AND RWYS. VISIBILITY WAS 3 MI. WINDS 150 DEGS AT 15 KTS. IT TOOK NEARLY AN HR TO DEICE US. WE BLOCKED OUT AT XA32Z. WE WERE CLRED TO TAXI TO RWY 12R VIA TXWY K; RWY 13 AND 31; TXWY F TO TXWY M. THE TAXI SURFACE WAS SNOW COVERED AND THE CTRLINE AREAS WERE SANDED. THE INITIAL TAXI PHASE WAS NORMAL; BUT AS WE APCHED RWY 17 AND 35; THE CAPT ACCELERATED (POSSIBLY OUT OF HABIT WHEN XING ANOTHER RWY) BUT HE DID NOT SLOW IT DOWN ONCE WE HAD CROSSED. AS WE APCHED THE TURN OFF TXWY K ONTO RWY 13 AND 31; I FELT WE WERE TOO FAST. WE BEGAN THE TURN AND THE ACFT SEEMED TO TAKE THE TURN WELL. AS WE MADE THE TURN; I STATED I FELT WE WERE TOO FAST AND WE NEEDED TO SLOW DOWN. THE CAPT OVERSHOT THE TURN AND WE WERE WELL L OF CTRLINE. HE CORRECTED BACK TO THE R AND THE ACFT STOPPED RESPONDING TO INPUTS. WE WERE SLIDING R. THE CAPT ATTEMPTED TO ARREST THE SLIDE BY USING REVERSE THRUST; BUT IT WAS INEFFECTIVE. THE ACFT CAME TO REST WITH THE NOSEWHEEL APPROX 18 INCHES OFF THE ASPHALT. THE MAIN GEAR REMAINED ON THE ASPHALT. THE CAPT SUGGESTED USING REVERSE THRUST TO BACK OUT; BUT I STATED I DID NOT THINK THAT WAS A GOOD IDEA. THE ACFT WAS SUBSEQUENTLY TOWED OUT. THERE WAS NO DAMAGE TO THE PLANE. WHEN I ASKED THE CAPT WHY HE LET THE ACFT ACCELERATE; HE STATED HE DIDN'T WANT TO GET 'BOGGED' DOWN IN THE SNOW. LESSONS LEARNED: 1) AT THE FIRST FEELING OF DISCOMFORT; SAY SOMETHING. 2) WHEN OPERATING IN UNUSUAL CONDITIONS; TALK ABOUT THE PLAN OF ACTION. IF THE CAPT HAD TOLD ME HE PLANNED TO KEEP THE SPD UP ON THE TAXI; I WOULD HAVE VETOED THE IDEA BASED ON THE CONDITIONS. 3) DON'T ASSUME. I ASSUMED WE WOULD BE TAXIING SLOWLY. THE CAPT HAD OTHER PLANS. 4) GET A GOOD BRAKING ACTION RPT ON YOUR TAXI RTE. THE OTHER SIDE WAS GOOD; BUT THE CARGO SIDE WAS NOT. WE TOOK TXWY BRAKING ACTION AS GOOD TO MEAN ALL TXWYS. THE ONE WE SLID OFF OF WAS ONE HALF INCH ICE COVERED BY SNOW.