Driving on the left or right
 Sign announcing change to driving on the right, Ghana, 1974
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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.
The following terms are used in this article:
Ancient Greek, Egyptian, and Roman troops kept to the left when marching. Which side of the road the Ancient Romans drove on is disputed. Roman roads in Turkey suggest Romans used the right-hand side of the road. Archaeological evidence in England suggests driving on the left. In 1998, archaeologists found a well-preserved double track leading to a Roman quarry near Swindon. The grooves in the road on the left side (viewed facing down the track away from the quarry) were much deeper than those on the right side. These grooves suggest that the Romans drove on the left, at least at this location, since carts would exit the quarry heavily loaded, and enter it empty.
The first reference in English law to an order for traffic to keep to the left was in 1756, with regard to London Bridge. The General Highways Act of 1773 reportedly contained a recommendation that horse traffic should keep to the left.
Some historians, such as C. Northcote Parkinson, believed that ancient travellers on horseback or on foot generally kept to the left, since most people were right handed. If two men riding on horseback were to start a fight, each would edge toward the left. In the year 1300, Pope Boniface VIII directed pilgrims to keep left.
In the late 1700s, traffic in the United States was on the right was based on teamsters' use of large freight wagons pulled by several pairs of horses. The wagons had no driver's seat, so a postilion sat on the left rear horse and held his whip in his right hand. Seated on the left, the driver preferred that other wagons pass him on the left so that he could be sure to keep clear of the wheels of oncoming wagons.
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