A violent highway attack by a lone gunman has caused Australian authorities to warn travelers to avoid three major highways in the north central part of the country. A British couple was held up by a man who signaled that he had car trouble July 14 on the Stuart Highway near Barrow Creek, 155 miles north of Alice Springs. Highways to avoid are the Stuart Highway between Darwin and Port Augusta, the Victoria Highway between Katherine and Kununurra in Western Australia, and the Barkly Highway between Tennant Creek and Mt. Isa in Queensland. Authorities have warned not to stop for anyone requesting help.