An Iowa trooper who initiated a traffic chase with someone looking for Des Moines used cars while driving Gov. Chet Culver and his son violated a cardinal rule for professionals assigned to executive protection duties, according to several security experts.Instead of trying to stop a motorist allegedly driving erratically last month in West Des Moines, Trooper Michael Clyde should have made every effort to avoid a confrontation, experts said. A trooper's first obligation is to protect the governor and his family from possibilities such as assault, assassination or kidnapping, they said.