The general rule of the road was set for Britain and North America at least a century before the automobile, though there was so little traffic in most places that it just wasn't needed all that often, particularly in cities where streets were wide. In many less-developed nations, a rule wasn't even set down until the middle of the 20th century.

At any rate, in North America, the task of keeping out of the ditch or swamp was more important early on than the task of avoiding other vehicles. The definitive reference work on this subject is Peter Kincaid's 1986 book The Rule of the Road: An International Guide to History and Practice.

Writes Kincaid: "In summary, different types of transport, all used by right-handed people, tended to produce different rules of the road. Armed walkers and armed horsemen tended to keep left to leave their swordarms free. Horse riders kept left in any case because they mounted from the left and stayed near the edge where it was easier and safer to mount and dismount than in the middle of the road. People leading horses with their right hands tended to keep right because the led horse was then protected from passing traffic. Carters tended to keep right because they walked on the left side of their horses, leading with the right hand, and by keeping right could walk in the middle of the road...to avoid collisions. Postilion riders tended to keep right because they sat on the left-rear horse and thus could better judge clearances....Drivers who sat on the vehicle kept left because they sat on the right to keep their whip hands free and could judge clearances better when passing if they kept left." Kincaid describes other contributing factors such as conformance with neighbors (undoubtedly the reason for Canada), influence of colonization, national unity, imported vehicles, etc. Although we tend to think of a keep-left rule requiring right-hand controls, and vice versa, he points out a number of instances where curbside controls have been preferred to centerline controls.

As of 1986, he counted 118 "independent territories" with right-hand traffic and 51 with left-hand, adding: "The above figures show what a minority rule left-hand traffic is today. Countries which use it account for only about a third of the world's population, a sixth of its area, a quarter of its roads, and a sixth of its motor vehicles." A number of countries have changed their rule of the road, including, since 1950: Cameroon, Belize, Ethiopia, Sweden, Bahrain, Iceland, Burma, Sierra Leone, Nigeria, Ghana, and South Yemen. All these but Burma changed from left to right. British Columbia swapped from left to right in 1921—but Vancouver didn't swap until Jan 1, 1922 because all the streetcar infrastructure made it difficult.

The American expert on this subject is Richard H. Hopper, whose article "Left-Right: Why Driving Rules Differ," appeared in Transportation Quarterly 36 (1982), pp. 541-548.