ACR X STARTED GAR BECAUSE OF NO LNDG CLRNC THEN LANDED WHEN LNDG CLRNC ISSUED.

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ACR X DCA BOS WX CONDITIONS AT BOS WERE SUCH THAT CATII OPS WERE IN WE (1200-2000 RVR) BRIEFING ON APCH WAS GIVEN AS PER COMPANY SOP. THE APCH WAS A DUAL CHANNEL AUTOPLT WITH AUTOLAND TO RWY 4R BOS. BOSTON TWR ADVISED THAT THE LNDG CLRNC WOULD BE DELAYED BECAUSE A TRUCK WAS ON THE RWY AND 'HE WAS ALREADY HALFWAY DOWN THE RWY.' THE RWY ENVIRONMENT APPEARED AT APPROX 150 FT AGL AND I TOOK CTL OF THE ACFT AS PER THE BRIEFING. I ANNOUNCED 'LNDG' AND THE FO REMINDED ME WE HAD YET TO BE CLRED BY TWR. THERE WAS QUITE A BIT OF 'CLUTTER' ON THE FREQ AND I THOUGHT I HEARD 'CLRED TO LAND' BUT WE COULD NOT VERIFY THIS BECAUSE OF THE CLUTTER. A GAR WAS INITIATED ON THE AUTOPLT. AS THE THROTTLES ADVANCED; CONTACT WAS MADE WITH THE MAIN WHEELS AND WE BECAME AIRBORNE AGAIN AS TWR CLRED US TO LAND. THE THROTTLES WERE CLOSED AND THE LNDG WAS MADE WITHOUT INCIDENT. I BELIEVE THE MISSED APCH SHOULD HAVE BEEN MADE OR INITIATED SOONER ON MY PART. ADDING TO THE PROBLEM WAS THE SEPARATION ON THE APCH; THE NUMEROUS NEEDLESS XMISSIONS ON TWR FREQ AND THE PRESENCE OF A VEHICLE ON THE ACTIVE RWY DURING THE CATII APCHS.