DRIVER SIDE CURTAIN AIR BAG DEPLOYED FOR NO APPARENT REASON. TRAVELING EAST ON I-94, IN MILWAUKEE, WI, ABOUT 9200 EAST. CONDITIONS: 12:45 PM, 37 DEGREES, LIGHT RAIN. WE HAD BEEN DRIVING ABOUT 20 MINUTES. ENGINE WAS COMPLETELY WARM. NO ALCOHOL WAS INVOLVED. DRIVER SUFFERED A SHOULDER INJURY FROM THE FORCE OF THE AIRBAG, BUT REFUSED TREATMENT. DRIVER MANAGED TO RETAIN CONTROL AND PULLED OVER ON THE RIGHT BERM. HE INSPECTED EXTERIOR OF VEHICLE. THERE WAS NO INDICATION OF ANYTHING HITTING THE CAR. HE THEN CONTINUED TO DRIVE HOME WITHOUT INCIDENT. *TR