A320 CREW SHORTLY AFTER ROTATION; IN A LOW VISIBILITY CONDITION; SAW VEHICLES ON THE RWY.

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TAXI FOR TKOF IN RESTR VISIBILITY AT LGA FOR DEP ON RWY 4. GND CTL AND TWR WERE COMBINED ON 1 FREQ. AN ACR X ACFT WAS WAITING FOR TKOF. JUST AFTER BEGINNING OUR TAXI FROM GATE; WE HEARD WHAT SOUNDED LIKE A SNOW PLOW ASK IF HE COULD HAVE THE RWY BACK AFTER ACR X DEPARTED. TWR SAID YES. WE CALLED THE TWR TO REMIND THEM THAT WE WERE TAXIING OUT SO THEY WOULD KEEP THE RWY OPEN LONG ENOUGH FOR US TO DEPART. THE FO THEN CONTACTED DISPATCH TO UPDATE THEM ON FIELD CONDITIONS. I HEARD A VEHICLE REQUEST CLRNC TO CROSS THE RWY; WHICH WAS GRANTED. TWR TOLD US TO CALL WHEN #1 FOR TKOF. WE DID -- ABOUT 3-5 MINS AFTER I HEARD THE VEHICLE ASK FOR XING CLRNC. WE THEN GOT CLRNC TO TAXI INTO POS AND HOLD TO CALL WHEN READY. WE CALLED READY AND WERE CLRED FOR TKOF. JUST AFTER ROTATION WE SAW VEHICLES XING OUR RWY AT ABOUT TXWY P OR TXWY G. WE FLEW OVER THEM AT 50-75 FT (CEILING 100 FT). I RPTED THIS TO TWR. THE CTLR ASKED THE VEHICLES WHAT THEY WERE DOING ON HIS RWY. I THINK EITHER THE CTLR ASSUMED THEY HAD ALREADY CROSSED (THEY SHOULD HAVE BEEN CLR SEVERAL MINS EARLIER) AND I ALSO THINK THE SNOW PLOWS THOUGHT ACR X WAS THE ONLY DEP AND THEY COULD HAVE THE RWY BACK. APPARENTLY; ONLY 1 CAR HAD AN ATC RADIO AND THE OTHER PLOWS AND VEHICLE FOLLOWED HIM; OR THEY JUST CROSSED WITHOUT CLRNC. THEY SHOULD HAVE HEARD OUR TKOF CLRNC. THEY KNEW WE WERE TAXIING BECAUSE THEY HAD TO GIVE WAY TO US AT TXWY E.