LTT CREW HAD RWY TXWY EXCURSION DUE NIL BRAKING AT AMA.

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LANDED AT AMA ON RWY 22. BRAKING ACTION RPTED AS FAIR ON ATIS. PREVIOUS ACR LANDED RPTING BRAKING ACTION POOR ON THE TXWY. RWY AND TXWYS WERE OBSCURED WITH LIGHT COVERING OF SNOW. SLOWLY BEGAN TURN TO EXIT RWY ON TXWY A. BRAKING ACTION TURNED TO NIL BTWN RWY CTRLINE AND THE TXWY. DIRECTIONAL CTL COULD NOT BE MAINTAINED AND ACFT BEGAN TO SLIDE. THE NOSEWHEEL SLID INTO THE GRASS AND THE 2 MAINS STAYED ON THE PAVEMENT. THE ENGS WERE SHUT DOWN. TWR; THE FLT ATTENDANT; COMPANY; DISPATCH; AND THE PAX WERE NOTIFIED. THE ARPT VEHICLES AND BUS ARRIVED AT THE AIRPLANE. ALL THE VEHICLES WERE SLIDING ON THE RWY DUE TO THE ICE. THE PAX DEPLANED AND WERE BROUGHT TO THE TERMINAL. A STATEMENT WAS MADE BY A TUG DRIVER THAT THERE WAS 'BLACK ICE' ON THE ARPT SURFACE. AN ACR CAPT STATED HE HAD DIFFICULTY MANEUVERING ON THE RAMP DUE TO ICE. I BELIEVE THE ARPT FACILITY NEEDS TO IMPROVE THEIR STANDARDS OF REMOVING ICE AND SNOW FROM THE ARPT SURFACE. SUPPLEMENTAL INFO FROM ACN 458275: LNDG ON RWY 22 AT AMA WAS NORMAL. BRAKING ACTION WAS RPTED AS FAIR AND RWY AND TXWYS WERE OBSCURED WITH A LIGHT COVERING OF SNOW. DURING ROLLOUT; I APPLIED VERY LITTLE BRAKING AND USED ONLY GND IDLE TO SLOW ACFT. I BELIEVED WE WERE GOING TOO FAST TO MAKE TXWY B; SO I CONTINUED TO TXWY A. APCHING TXWY A; I BEGAN TO BRAKE AND EXIT THE RWY. BRAKING ACTION RAPIDLY BECAME NIL AS WE ENCOUNTERED A LARGE PATCH OF ICE. THE ACFT BEGAN TO SLIDE. NOSEWHEEL STEERING WAS UNABLE TO CHANGE DIRECTION OF ACFT AND WE SLID OFF THE RWY AT THE INTXN OF THE TXWY. THE ACFT CAME TO A REST WITH THE NOSE GEAR ON THE GRASS AND BOTH MAINS ON RWY/TXWY PAVEMENT. BOTH ENGS WERE SHUTDOWN. ATC; FLT ATTENDANT; PAX AND COMPANY WERE INFORMED OF SIT. AFTER LNDG AND PARKING; CHKS WERE COMPLETED. PAX WERE THEN DISEMBARKED. UPON ARR OF ARPT VEHICLES; I NOTED (OFF THE R SIDE OF ACFT) THAT THEY WERE SLIDING ON THE ICE AND HAVING CTL PROBS. IT APPEARED THAT THE ICE EXTENDED FROM OUR POS TO ABOUT 40 FT TOWARDS RWY CTRLINE. THIS INCIDENT HAS SHOWN ME HOW INACCURATE BRAKING ACTION RPTS ARE. I ALSO BELIEVE THAT ARPT PERSONNEL SHOULD MAKE HRLY INSPECTIONS OF ALL RWYS AND TXWYS WHEN CONDITIONS WARRANT A BRAKING ACTION RPT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: ACFT DEPARTED HARD SURFACE AT A SPD NOT FASTER THAN A SLOW WALK. ACFT WAS NOT DAMAGED. FLT CANCELED TO ALLOW ACFT MAINT CHKOUT. ARPT AUTHS MADE NO COMMENT ON THE POORLY PREPARED ARPT SURFACES.