The terms right-hand traffic and left-hand traffic refer to regulations requiring all bidirectional traffic, unless otherwise directed, to keep to the right or to the left side of the road, respectively. This is so fundamental to traffic flow that it is sometimes referred to as the rule of the road. Today, about 65 percent of the world's population live in countries with right-hand traffic and 35 percent in countries with left-hand traffic. About 90 percent of the world's total road distance carries traffic on the right and 10 percent on the left. Right-hand traffic predominates across most of the continental landmasses, while the majority of the world's island nations and territories drive on the left.