THE PLT & PASSENGER WERE PARTICIPATING IN THE CAFE 400 EFFICIENCY RACE. SHORTLY AFTER PASSING A CHECK POINT, A PORTION OF 1 PROPELLER BLADE SEPARATED & THE PLT SHUT DOWN THE ENG. HE ELECTED TO LAND ON AN INTERSTATE HIGHWAY BY MERGING INTO TRAFFIC. HOWEVER, HE WAS UNABLE TO CLEAR OVER AN OVERPASS & HAD TO GO UNDER. SUBSEQUENTLY, THE ACFT TOUCHED DOWN EARLY. BEFORE STOPPING, THE CANARD HIT & SCRATCHED AN AUTOMOBILE & THE ACFT COLLIDED WITH THE CENTER DIVIDER OF THE HIGHWAY, COLLAPSING THE NOSE GEAR & DAMAGING THE CANARD. THE PROPELLER WAS MADE FROM WOOD & WAS SPECIFICALLY DESIGNED FOR RACING. THE TIP & OUTBOARD TRAILING EDGE OF THE FAILED BLADE HAD SPLIT & SEPARATED. REPORTEDLY, THE PROPELLER HAD BEEN FLOWN THRU DRIVING RAIN AT FULL POWER. ACCORDING TO A SERVICE MANAGER OF A PROPELLER MANUFACTURING FIRM, THE BLADE FAILURE MAY HAVE ORIGINATED WHERE IT WAS STRUCK BY AN OBJECT, BUT THIS WAS NOT VERIFIED.