NMAC AFTER UNAUTH RWY ENTRY OP AT A NON TWR ARPT.

Narrative

TAXIING OUT OF BIG BEARS TERMINAL AREA I APCHED WHAT I ASSUMED WAS THE TXWY TO RWY (08?) (07). I LOOKED CLOSELY FOR RWY SIGNS OR THE HOLD SHORT STRIPES AND SEEING NONE I TAXIED ONTO WHAT I THOUGHT WAS THE TXWY LEADING TO THE RWY. AS I TAXIED OUT I HEARD AN ACFT ANNOUNCE IT WAS TAKING OFF ON RWY 08 (07). I INSTINCTIVELY LOOKED DOWN TOWARD THE RWY END AS I MADE THE TURN ONTO WHAT TURNED OUT TO BE THE RWY. THERE IS A MILD RISE AT THE APCH END OF RWY 08 WHICH STILL MADE IT CONFUSING AS TO WHETHER THE ACFT AND I WERE ON THE SAME CONCRETE. WITHIN SECONDS THE ACFT TOPPED THE RISE AND THERE WAS NO QUESTION I WAS ON THE RWY. I TAXIED THE L MAIN INTO THE DIRT ON THE SIDE OF THE RWY WHILE THE TWIN WAS AIRBORNE AT 30 FT AND PASSED APPROX 10 FT OFF TO THE L. I RAPIDLY DID A 180 DEG TURN AND TAXIED OFF THE RWY. THIS TIME PAYING PARTICULAR ATTN TO SEE IF THERE WERE ANY MARKINGS OF AN ACTIVE RWY; I NOTED THAT THERE WERE NO SIGNS BUT THAT A YELLOW RWY HOLD SHORT LINE HAD BEEN PAINTED ON THE TXWY. THIS LINE; EXCEPT AT THE EDGES HAD BEEN COVERED WITH DIRT AND DUST TO THE POINT WHERE IT WAS ALL BUT UNSEEABLE. (THERE IS NO INSTRUMENT APCH TO BIG BEAR.) I FEEL THERE IS POTENTIAL FOR FUTURE RWY INCURSIONS AT ARPTS WHERE: TAXI AND RWY PAINTED MARKINGS ARE NOT CLEANED AND REPAINTED AT REGULAR INTERVALS. RWY AND TXWY SIGNS ARE (NOT) CLRLY POSTED.