WHILE TRAVELING SOUTH ON I-275 IN ST PETERSBURG, FLORIDA, IN THE MIDDLE LANE WITH LIGHT TRAFFIC, IN CLEAR WEATHER WITH ACCELENT VISIBILITY, AT BETWEEN 60 AND 65 MILES PER HOUR, THE PASSENGER SIDE, SIDE IMPACT AIRBAG SUDDENLY DEPLOYED. THE EXPLOSIVE DEPLOYMENT SPRAYED SHARDS OF HARD PLASTIC THROUGHOUT THE PASSENGER COMPARTMENT. THERE WAS NO APPARENT DAMAGE TO THE VEHICLE. THE PROBLEM WAS CAUSED BY AN IMPACT OF A SMALL PROJECTILE, PROBABLY A MEDIUM SIZE PEBBLE THROWN UP FROM THE BOTTOM OF THE FRONT RIGHT WHEEL, STRIKING THE BOTTOM OF THE VEHICLE, WHICH SENT A FALSE SIGNAL OF A SIDE IMPACT TO THE SIDE IMPACT SENSOR. THE BOTTOM OF THE VEHICLE WAS CLEAN AND FREE OF ANY DAMAGE EXCEPT FOR A SLIGHT INDENTATION ALMOST DIRECTLY BEHIND THE LEFT SIDE OF THE RIGHT FRONT TIRE, A FEW FEET BEHIND THE FRONT RIGHT TIRE. THE DAMAGE AREA CONSISTED OF AN INDENTATION IN THE BOTTOM OF THE CAR; IT MEASURED APPROXIMATELY 1.5 INCHES IN LENGTH BY .33 INCHES IN WIDTH, RUNNING LENGTHWISE TO THE VEHICLE. THE INDENTATION WAS ROUNDED; THERE WAS STILL WHAT APPEARED TO BE UNDERCOATING MATERIAL COATING THE INDENTATION. THE THIN GALVANIZED METAL COVERING THE SENSOR APPEARS TO BY DESIGN PROTRUDE DOWN AS IF TO WRAP AROUND THE SENSOR BAR IN THIS AREA. THIS MAKES IT MORE SUSCEPTIBLE TO DAMAGE. THE PROBLEM WAS CLEARLY CAUSED BY EITHER MALFUNCTION OF, OR A DESIGN DEFICIENCY OF THE SENSOR AND ITS MOUNTING. A SIDE IMPACT SENSOR SHOULD IN FACT MEASURE MOTION LATTERLY, SIDE TO SIDE. THE SENSOR DID NOT DO THIS; IT