AFTER LNDG AT LGB; C150 TAXIED TOWARD RAMP AND CROSSED AN ACTIVE RWY WITHOUT CLRNC.

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AFTER DEPARTING SNA ARPT AND FLYING UNEVENTFULLY UP THE COAST; I ACQUIRED THE CURRENT ATIS AT LGB ARPT; CONTACTED THE TWR AND ENTERED THEIR AIRSPACE; JOINED L TFC AND AFTER A VERY EXTENDED DOWNWIND (TWR CALLED MY BASE); I LANDED ON RWY 16L. TWR ASKED MY DEST ON THE ARPT AND I REPLIED AN FBO ON THE W SIDE. TWR THEN TOLD ME TO TURN R FROM THE ACTIVE RWY 16L ONTO AN INACTIVE RWY 25L; WHICH I DID. I'M NOT FAMILIAR WITH LGB ARPT BUT KNEW THERE WERE 5 XING/INTERSECTING RWYS (INCLUDING A RWY 16R STAGGERED FROM RWY 16L). TWR DIDN'T ADVISE ME TO CONTACT GND (AS I AM USED TO) AND SINCE I WAS TAXIING ON AN INACTIVE RWY; I ASSUMED THEY WANTED ME ON THEIR FREQ WHERE I REMAINED. I CONTINUED WBOUND ON RWY 25L LOOKING FOR THE FBO AND UNKNOWINGLY CROSSED ACTIVE RWY 16R WITHOUT A CLRNC AS AN ACFT WAS COMPLETING A LNDG ROLL! I BELIEVE SEVERAL FACTORS COMPOUNDED TO THIS RESULT: 1) RWY 16R WASN'T MENTIONED AS IN USE IN THE ATIS. 2) AN AIRSHIP (TO THE W OF ME ON FINAL) WAS ALSO ON FINAL WITH A DEST BTWN THE RWYS; MAKING ME BELIEVE EVEN MORE SO THAT RWY 16R WAS INACTIVE. 3) TAXIING ON AN INACTIVE RWY (IE; NO HOLD SHORT LINES INDICATING AN INTERSECTING RWY WHICH I USUALLY KEY ON). 4) NEVER HEARING FROM THE TWR TO EITHER HDOF TO GND OR TO HOLD SHORT OF RWY 16R. 5) SEEING THE OTHER ACFT IN THE DISTANCE; BUT ASSUMED IT WAS ALSO TAXIING. TWR WAS BUSY; BUT IN RETROSPECT; I SHOULD HAVE VERIFIED INTENTIONS WITH THEM BEFORE THAT MUCH TIME ELAPSED. STILL DON'T KNOW WHY THEY DIDN'T CALL ME (PROBABLY WERE VIA GND!) BEFORE WE RESULTED IN A LOSS OF SEPARATION; BUT I'LL KNOW SOON -- I'VE BEEN INVITED TO A 'PLTDEV' INQUIRY! MORAL: DON'T ASSUME. I THINK WE ALL KNOW WHAT IT DOES.