SMA WITH SPI HAS ENG OVERHEAT. FORCED LNDG ON HWY. CAUSE: FAILURE TO REMOVE COWL PLUGS.

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WHILE AT 2000 FT OVER RAPID RIVER; I NOTICED THE TEMP GAUGE APCHING RED LINE. I IMMEDIATELY LOOKED FOR A FIELD TO LAND ON. THE CLOSEST LARGE ENOUGH FIELD THAT APPEARED SUITABLE ON SHORT FINAL I NOTED IT WAS WATER SOAKED WITH PUDDLES AND ABORTED THE ATTEMPT. ON PULLING AWAY FROM THE FIELD ON FULL PWR I AGAIN NOTED I WAS NEARLY RED LINED ON THE TEMP GAUGE. U.S. 2 WHICH RUNS NE FROM GLADSTONE TO RAPID RIVER IS A 4 LANE DIVIDED HWY WITH LIGHT TFC. KNOWING I COULD NOT MAKE THE ESCANABA ARPT WHICH WAS ABOUT 12 MI FURTHER S; I ELECTED TO LAND ON THE HWY. AFTER LNDG SAFELY; I SHUT DOWN THE ENG AND PUSHED THE PLANE'S TAIL OVER THE SHOULDER WITH THE WINGS PARALLEL TO THE SHOULDER AND CLR OF THE HWY. INSPECTION OF THE ENG REVEALED I HAD FAILED TO REMOVE THE COWL PLUGS WHICH PREVENTED AIR COOLING. THE SOLUTION IS OBVIOUS -- REMOVE COWL PLUGS BEFORE FLYING. ALSO; ATTACHING RED RIBBONS TO THE COWL PLUGS WILL ASSIST IN NOTING THEY ARE STILL IN THE COWL. AFTER COOLING THE MOTOR; THE STATE POLICE ON HAND HELD BACK THE TFC AND THE TKOF AND RETURN TO THE ESCANABA ARPT WAS UNEVENTFUL. MY LCL MECH CHKED THE MOTOR AND FOUND NO PROBS. I BELIEVE MY PROMPT ACTION IN LNDG AND NOT TRYING TO MAKE THE ARPT KEPT THE ENG FROM SEIZING AND POSSIBLY HAVING TO LAND WHERE THERE WAS NO SUITABLE PLACE TO DO SO. LESSON LEARNED: BE MORE OBSERVANT ON PREFLT.