CAPT LNDG AN MLG MISSED HIGH SPD TURNOFFS FROM RWY 27 AT BOS; OVERSHOT TAXIWAY S BUT MANAGED TO TURN ACFT SHARPLY AND REGAIN TAXIWAY S. DELAY IN CLRING RWY CAUSED A GAR OF ANOTHER ACFT.

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UPON LNDG ON RWY 27 AT LOGAN; WE MISSED THE HIGH SPD TURNOFFS (E AND X) AND WERE THEN INSTRUCTED BY LCL E TO 'EXPEDITE TO THE END AND MAKE A R TURN AT THE BLUE LIGHTS.' AS WE APCHED TAXIWAY SIERRA; I REALIZED THAT THE CAPT WAS LOOKING DOWN THE RWY OVERRUN TO THE RED THRESHOLD LIGHTS AS BEING 'THE END' AND THE TAXIWAY LIGHTS ON B AS BEING THE 'BLUE LIGHTS' MENTIONED BY LCL. THE CTLR HAD ALSO INSTRUCTED US TO 'HOLD SHORT OF 22R AND MONITOR TWR ON 128.80; COMPANY ON SHORT FINAL.' I HAD ASSUMED; INCORRECTLY; THAT THE CAPT UNDERSTOOD THAT WE WERE EXPECTED TO TURN R ON SIERRA AND MADE THE MISTAKE OF LOOKING DOWN AT THE CENTER CONSOLE TO PRELOAD 128.80 IN THE STANDBY COM REGISTER. AS I LOOKED UP (AND SWITCHED TO 128.80); I REALIZED THAT THE CAPT WAS GOING TO ROLL THROUGH SIERRA AND INTO THE CHEVRON MARKED STOPWAY (WHICH WAS ONLY VERY DIMLY VISIBLE AS BEING SO MARKED); SO I CALLED 'STOP' AND POINTED OUT SIERRA AS BEING OUR CLRED RTE. THE CAPT STOPPED THE ACFT WITH THE NOSEWHEEL JUST PAST THE LINE OF BLUE LIGHTS ACROSS 27 DELINEATING THE WESTERN EDGE OF SIERRA. HE THEN MADE A HARD R TURN RUNNING THE NOSEWHEEL BTWN THE BLUE LIGHTS AND BACK ON TO SIERRA (THE MAINS HAD REMAINED ON THE RWY STRADDLING THE CENTERLINE) AND PROCEEDED NBOUND ON SIERRA AS INSTRUCTED. I LOOKED BACK DOWN THE RWY AND SAW THE COMPANY ACFT EXECUTING A GAR BUT SINCE I HAD ALREADY SWITCHED TO LCL W; DID NOT HEAR THE EXCHANGE BTWN THE CTLR AND THE OTHER ACFT. I BELIEVE THAT THE MARKINGS AND LIGHTING AT THIS INTXN ARE A MAN-TRAP AND GIVEN THE INSTRUCTIONS WE RECEIVED; THE CAPT'S INTENDED COURSE OF ACTION WAS LOGICAL AND A VERY EASY MISTAKE TO MAKE. I WOULD ENCOURAGE MASSPORT TO RELOCATE THE RED THRESHOLD LIGHTS TO THE W EDGE OF SIERRA. BARRING SOME SUCH REMEDIAL ACTION; HIS ERROR IS LIKELY TO RECUR.