ACFT WITH NO FLT PLAN ENTERED MOUNTAIN PASS; WX DETERIORATED; BECAME BOXED IN AND LANDED ON INTERSTATE HWY.

Narrative

FLT WAS ENRTE FROM BILLINGS; MT; TO SPOKANE; WA. WX FORECAST FROM BILLINGS FOR THE RTE WAS LOWEST 6000 FT SCATTERED WITH 5 MI IN LIGHT RAIN AND FOG AND NO AIRMETS. AT MISSOULA I CHKED WX IN FLT WITH GREAT FALLS FSS. NEW WX AIRMET FOR OUR RTE PARTIAL MOUNTAIN OBSCURATION AND LOWEST CEILING AND VISIBILITY RPTED AND FORECAST WAS 6000 FT BROKEN AND 5 MI DUE TO LIGHT RAIN. THE WX DETERIORATED CLOSE TO MULLAN PASS BUT I DECIDED I HAD AMPLE WX TO FLY OVER THE PASS. 5 MI W OF MULLAN; ID; THE CLOUDS HAD OBSCURED THE VALLEY TO A POINT THAT I FELT IT WAS UNSAFE TO PROCEED WBOUND TO SPOKANE. I TURNED AROUND TO HEAD BACK TOWARD MISSOULA TO LAND ONLY TO FIND THAT MULLAN PASS WAS NOW COMPLETELY OBSCURED BY FOG. I WAS TRAPPED IN THE VALLEY AND CIRCLED MULLAN FOR ABOUT 20 MINS AND DECIDED TO LAND ON THE FREEWAY BECAUSE THE CEILING AND VISIBILITY WAS LOWERING. I LANDED ON INTERSTATE 90 WITHOUT DAMAGE OR HARM TO THE PLANE; ROAD; VEHICLES OR PAX. HINDSIGHT; YES I SHOULD HAVE TURNED AROUND BEFORE MULLAN PASS AND RETURNED TO MISSOULA TO WAIT OUT THE WX. NOW; I SEE THAT THAT WAS THE MOST REASONABLE CHOICE FOR SAFETY ESPECIALLY IN UNFAMILIAR TERRAIN. THE REASON FOR CONTINUING WAS THE PUSH TO COMPLETE THE TRIP. I GUESS MY JUDGEMENT WAS IMPAIRED DUE TO EXHAUSTION FROM THE LENGTH OF THE TRIP (SINCE IT HAD ORIGINATED FROM ENID; OK). I HAD 2 FUEL STOPS IN RTE FROM ENID AND HAD ABOUT 6 HRS FLYING TIME TO THE POINT OF LNDG ON THE FREEWAY.