The terms right-hand traffic and left-hand traffic refer to regulations requiring all bidirectional traffic, unless otherwise directed, to keep either to the right or 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. This basic rule improves traffic flow and reduces the risk of head-on collisions. Today, about 65% of the world's population lives in countries with right-hand traffic and 35% in countries with left-hand traffic. About 90% of the world's total road distance carries traffic on the right and 10% on the left.