SMALL AIRCRAFT PILOT REPORTS RAPIDLY DETERIORATING WEATHER WHICH BLOCKS ATTEMPTS AT DIVERSION AND RESULTS IN SAFE LANDING ON STATE HIGHWAY.

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I WAS ENRTE FROM THE SLC AREA TO DIJ. I EVALUATED THE WX OVER SODA SPRINGS; ID (U78) AND DECIDED THAT IT WAS VFR TO DRIGGS; ID (DIJ). WHILE ENRTE BTWN SODA SPRINGS AND DIJ; THE WX STARTED TO COME DOWN. AT ABOUT THE HALFWAY POINT; I DECIDED TO TURN 180 DEGS FOR AFO BY WAY OF SODA SPRINGS (U78). ONCE ENRTE FOR AFO AT THE TOP OF TIN CUP PASS; THE WX CAME DOWN MORE AND IT WAS NO LONGER VFR TO AFO. I TURNED 180 DEGS AGAIN BACK TOWARD SODA SPRINGS ONLY TO HAVE THE WX COMING DOWN THERE ALSO. OVER THE COURSE OF THE NEXT 5 MINS; IT BECAME APPARENT THAT I WAS RUNNING OUT OF OPTIONS AND DECIDED TO MAKE AN EMER LNDG ON A STATE HWY. AT THAT POINT; THE CONDITIONS HAD COME DOWN TO ABOUT 1 MI; CEILING 500-750 FT. THE LNDG WAS SUCCESSFUL WITH NO DAMAGE. I PULLED THE PLANE OFF THE HWY AND GOT A RIDE FROM A PASSING MOTORIST. EARLIER THAT DAY; I HAD GOTTEN A STANDARD WX BRIEF AND WAS AWARE THAT WX WAS MOVING IN BUT THAT BY MY CALCULATIONS I WOULD HAVE BEEN AHEAD OF IT. ADDITIONALLY; THE RADAR INFO ON MY GARMIN 496 CONFIRMED MY CONCLUSIONS. HAVING FLOWN IN THE MOUNTAINS FOR MORE THAN 1000 HRS; I HAVE NEVER SEEN THE WX COME DOWN LIKE A BLANKET THE WAY IT DID THAT DAY. CONTRIBUTING FACTORS: OVERCONFIDENCE IN KNOWING THE TERRAIN AND THE LCL WX. I WAS TIRED FROM FLYING ALL DAY. NEXT TIME I WILL NOT PUSH THE WX.