THE ACFT ENGINE LOST POWER WHILE DESCENDING AFTER 1 HR OF FLT. DURING THE ENSUEING FORCED LNDG THE PLT SAID HE CHANGED HIS SELECTION OF LANDING SITE BECAUSE OF AN AUTO ON THE SELECTED SITE (A HIGHWAY). THE ACFT WAS LANDED BESIDE THE ROAD &A COLLISION WITH A DIRT BANK RESULTED. THE PLT STATED THAT HE DID NOT CHECK THE FUEL QUANTITY BEFORE TAKEOFF BY VISUAL EXAM OF THE TANKS. HE SAID THAT THE FUEL GAGES INDICATED APRX 1/2 FULL AT PRE-FLT. THE ACFT HOLDS 26 GALLONS IN 2 WING TANKS. ONLY 22.5 GALLONS OF THE 26 IS USABLE. THE UNUSABLE FUEL IS THE INACCESSIBLE FUEL WHILE THE ACFT IS MANEUVERING. THE FAILURE OF POWER OCCURRED WHILE DESCENDING FROM 4100 FT TO 2100 FT MSL. POST ACCIDENT EXAM DID NOT DISCLOSE ANY EVIDENCE OF A PRE-IMPACT MALFUNCTION. THE WX CONDITIONS REPORTED AT THE TIME AND IN THE AREA OF THE ACCIDENT DEPICITS THAT THE ENGINE WAS OPERATING IN CARB ICING CONDITIONS. THE PLT DID NOT DESCRIBE HIS USE OF THE ENGINE CONTROLS, PRIOR TO OR DURING THE POWER LOSS, IN HIS WRITTEN REPORT OR INTERVIEW