DURING A BANNER TOWING MISSION, THE ACFT WAS OBSERVED FLYING AT LOW ALT NEAR CALIFORNIA STATE FREEWAY 91. ACCORDING TO WITNESSES, THE BANNER COLLIDED WITH POWER LINES AFTER THE ACFT HAD CROSSED OVER THE LINES AT A RELATIVELY SLOW SPEED. THE BANNER & TOW CABLE/ROPE RELEASED FROM THE ACFT & THE BANNER CAME TO REST ON THE POWER LINES. WITNESSES REPORTED THAT WHEN THE BANNER HIT THE POWER LINES, THE ACFT'S FORWARD MOVEMENT APPEARED TO STOP, THEN THE ACFT DROPPED OFF TO ITS LEFT & DESCENDED INTO THE PATH OF AN ONCOMING TRACTOR-TRAILER TRUCK. THE ACFT HAD NEARLY RECOVERED FROM THE DESCENT WHEN IT IMPACTED THE FREEWAY. THE DRIVER OF THE TRACTOR-TRAILER & HIS WIFE EXITED THE LEFT SIDE OF THE VEHICLE WITHOUT INJURY; HOWEVER, THEIR SON, WHO EXITED THE RIGHT SIDE DURING THE EVACUATION, RECEIVED MINOR INJURIES. THE ACFT & TRACTOR WERE DESTROYED BY IMPACT & FIRE. THERE WAS NO EVIDENCE OF A MECHANICAL MALFUNCTION WITH THE ACFT OR TOWING EQUIPMENT PRIOR TO THE COLLISION WITH THE POWER LINES.