FLC TAXIES ACFT OFF INTENDED TAXI RTE INTO DIRT.

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TAXI TO RWY 01R FROM HANGAR AT SFO. CLRED TO TAXI VIA TXWY C TO TXWY R; HOLD SHORT OF RWY 28R. CLRED TO CROSS AND TO CONTINUE TAXI ON TXWY R TO TXWY S; TXWY A TO RWY 1R. TAXIED STRAIGHT AHEAD AS DEPICTED ON ARPT CHART. NOSE GEAR LEFT RWY 28 INTO DIRT (MAIN MOUNTS REMAINED ON RWY 28). EXTRACTION OCCURRED BY TUG WITH STRAPS ATTACHED TO MAIN MOUNTS. NO DAMAGE TO ACFT. VISUAL INSPECTION BY MAINT AND FLT CONTINUED WITH NO INCIDENTS. GREEN TAXI LIGHTS ON TXWY R INDICATE STRAIGHT AHEAD TAXI AS DOES ARPT DIAGRAM (ACTUALLY ABOUT 110 FT JOG TO R IS REQUIRED TO STAY ON TXWY R). BRIGHT CONSTRUCTION LIGHTS AND TERMINAL CREATE ILLUSION TXWY R STRAIGHT AHEAD. BLUE LIGHTS AT THROAT OF TXWY U AND RWY 28R CAN BE MISTAKEN FOR R SIDE OF TXWY R. TXWYS R AND U POORLY LIGHTED; IF LIGHTED AT ALL. DISTINCTIVE SQUARE YELLOW SIGN WITH BLACK DIAGONALS SHOULD BE PLACED ON L SIDE OF RWY 28R TO SHOW TXWY R IS NOT STRAIGHT AHEAD. TAXI INSTRUCTIONS SHOULD INCLUDE TAXI ON RWY 28R TO TXWY R OR RENAME DISJOINED PART OF TXWY R.