It finally occurred to me to wonder why Americans, and especially Canadians, drive on the Continental (right) side of the road instead of the UK/Commonwealth standard. I believe the UK practice was well established in the 18th century, so the US should have followed that convention. Canada was an English possession until 1867, so the Canadian deviance is even more puzzling. (When Newfoundland left the UK for Canada in 1945 they had to change driving habits.)
This article came up early, but is a bit vague ..
Today, most of the countries that adhere to left side driving are those that came under the influence of British rule during the 19th Century. It would seem likely then that the USA, having been once a British colony, would have retained the driving on the left rule. However, in his book The Rule of the Road: An International Guide to History and Practice (now out of print), author Peter Kincaid states that he could find no evidence that left side driving was ever widespread in the USA. He attributes this to the influence of European settlers used to driving on the right, and also the fact that vehicles such as carts and the postillion-controlled Conestoga Wagons were popular in the colony and favored right-side driving. However, there may have been some parts of the country that did adhere to left side rules for a time.
In Canada, the evidence is that Ontario and Quebec, which started out under French influence, always had right side driving. Other areas such as British Columbia and the Atlantic provinces, remained staunchly English in their influence and drove on the left. They switched to the right in the 1920s to conform with the rest of Canada and the USA."
There's a bit more here ...
... In addition, the French Revolution of 1789 gave a huge impetus to right-hand travel in Europe. The fact is, before the Revolution, the aristocracy travelled on the left of the road, forcing the peasantry over to the right, but after the storming of the Bastille and the subsequent events, aristocrats preferred to keep a low profile and joined the peasants on the right. An official keep-right rule was introduced in Paris in 1794, more or less parallel to Denmark, where driving on the right had been made compulsory in 1793.
Later, Napoleon's conquests spread the new rightism to the Low Countries (Belgium, the Netherlands and Luxembourg), Switzerland, Germany, Poland, Russia and many parts of Spain and Italy. The states that had resisted Napoleon kept left – Britain, the Austro-Hungarian Empire and Portugal. ...
... In the early years of English colonisation of North America, English driving customs were followed and the colonies drove on the left. After gaining independence from England, however, they were anxious to cast off all remaining links with their British colonial past and gradually changed to right-hand driving. (Incidentally, the influence of other European countries’ nationals should not be underestimated.) The first law requiring drivers to keep right was passed in Pennsylvania in 1792, and similar laws were passed in New York in 1804 and New Jersey in 1813.
Despite the developments in the US, some parts of Canada continued to drive on the left until shortly after the Second World War. The territory controlled by the French (from Quebec to Louisiana) drove on the right, but the territory occupied by the English (British Columbia, New Brunswick, Nova Scotia, Prince Edward Island and Newfoundland) kept left. British Columbia and the Atlantic provinces switched to the right in the 1920s in order to conform with the rest of Canada and the USA. Newfoundland drove on the left until 1947, and joined Canada in 1949.
I wonder if this understates the influence of Quebec and Louisiana on the adaptation of right hand driving in North America. Quebec was very dominant along the river routes of (future) American and the northeast region through the 17th and early 18th centuries, and Louisiana would have had a similar influence from the south.
Any Quebecois historians out there?