THE PILOT REPORTED THAT HE ENCOUNTERED TURBULENCE, AND AT ABOUT THE SAME TIME, THE ENGINE BEGAN RUNNING ROUGH. HE BEGAN TROUBLESHOOTING AND DISCOVERED THAT THE ROUGHNESS WAS DIRECTLY PROPORTIONAL TO THROTTLE MOVEMENT; WHEN THE THROTTLE WAS ADVANCED, THE ROUGHNESS INCREASED. HE BELIEVED THE PROPELLER CONTROL HAD NO EFFECT ON THE ROUGHNESS. HE REPORTED THAT THE ENGINE RPM BEGAN TO DROP REGARDLESS OF MANIFOLD PRESSURE. HE ELECTED TO LAND ON AN INTERSTATE HIGHWAY. DURING THE LANDING, THE AIRCRAFT STRUCK A MOTOR VEHICLE AND AN EMBANKMENT. AFTER LANDING, THE PROPELLER BLADES WERE FOUND IN THE FEATHERED POSITION. AN EXAMINATION OF THE PROPELLER SYSTEM REVEALED THE BLADE ACTUATING PIN ON ONE BLADE HAD SEPARATED FROM THE BLADE BUTT, AT A PRE-EXISTING CRACK. THE PROPELLER HAD BEEN OVERHAULED ON 1/6/78 AT 932.8 HRS. THE RECOMMEND OVERHAUL PERIOD WAS 1200 HRS OR 5 YRS, WHICHEVER WAS FIRST.