ACFT ATTEMPTED TO EXIT RWY ONTO ACCESS ROAD.

Narrative

ATC FACILITY: LGA TWR. LOCATION: NE END OF RWY 4. LNDG LGA AT LATE NIGHT; MAX LNDG WT; WX 200 FT; WX AT MINIMUMS; BRAKING ACTION RPTED FAIR. PLOWING IN PROGRESS. ASKED TWR TO RPT RWY CLR OF EQUIP. EQUIP RPTED CLR WHEN WE WERE AROUND 500 FT. BROKE OUT AT 300 FT. RWY HALF PLOWED. R HALF PLOWED L HALF NOT PLOWED. USED MAX REVERSE AND HVY BRAKING. FOUND BRAKING ACTION POOR. SLOWED TO TAXI SPD APPROX 1000 FT FROM END. ONLY 1 RWY EXIT PLOWED. BEGAN TURN ONTO EXIT. IMMEDIATELY REALIZED THAT PLOWED EXIT WAS NOT A TXWY BUT TOO LATE TO TURN BACK ONTO RWY. MAINS REMAINED ON RWY NOSEWHEEL APPROX 10 FT ONTO PAVED AND PLOWED VEHICLE ROAD. CALLED FOR TUG AND INFORMED TWR THAT WE WERE NOT CLR. FULL RESPONSIBILITY FOR ERROR IS MINE HOWEVER INCORRECT DECISION INFLUENCED BY THE FACT THAT ROADWAY WAS THE ONLY PLOWED AND ONLY VISIBLE EXIT. RWY PLOWED ONLY ON R HALF HAD THE EFFECT ON BRAKING OF NO PLOWING AT ALL SINCE THE L MAINS WERE BRAKING IN SEVERAL INCHES OF WET SNOW AND SLUSH WITH VERY LITTLE BRAKING ACTION. EACH TIME THE ANTI-SKID RELEASED ON THE L THERE WAS A CORRESPONDING RELEASE ON THE R. SUGGEST THAT IT WOULD BE SAFER IF TWR CLOSED THE RWY UNTIL PLOWING IS COMPLETE. ALTHOUGH THE ACCESS ROAD SEEMED PASSABLE TO THE PARALLEL TXWY WE RESISTED THE STRONG URGE TO CONTINUE ON IT ONCE THE ERROR WAS REALIZED.