Highway 401 begins at Highway 3 in Windsor, Ontario (not at the Michigan border, contrary to popular belief), and ends at the Quebec border in Eastern Ontario, 815 kilometres from its start point in Windsor. Construction began in 1939, but was stopped almost immediately owing to the outbreak of World War II. The first section of the highway, between Toronto and Oshawa, was opened in 1947 and designated Highway 2A. It was officially designated 401 in 1952. The section through Toronto quickly became an urban commuter road, rather than a long-distance bypass route as was originally planned, leading to extensive traffic jams. This problem was solved to some extent by implementing an express/local set-up similar to the new Dan Ryan Expressway in Chicago. By 1967 the highway had been widened from four lanes to 12 or more through Toronto and Mississauga. Today the entire 401 through the Greater Toronto Area varies from 10 to 20 lanes, and the stretch between Mississauga and Brock Road in Pickering is thought to be the world's longest continuous highway having 12 or more lanes. An extensive plan is currently underway by the Ministry of Transportation to widen the highway to at least six lanes for its entire length and to extend the 12-lane express/local system as far west as Guelph.
Major freeway junctions are located at Highway 402, Wellington Road and Highbury Avenue in London, Highway 403, Highway 8, Highway 6, Highway 407, Highway 410, Highway 427, Highway 27, Highway 400, Allen Road and the Don Valley Parkway in Toronto, Highway 404 and Highway 416.
In 1965 Premier John Robarts named the highway the Macdonald-Cartier Freeway, in honour of Sir John A. Macdonald and Sir George-Étienne Cartier, two of the most important Fathers of Canadian Confederation.
Today it is North America's busiest highway, with close to 500,000 weekday trips between Highway 400 and Highway 404. There are also 18 service stations located along the route, allowing motorists to access services without travelling far from the highway. It is one of the most important highways in Canada, as it connects the majority of the population of southern Ontario with all the other highways in the province, as well as all the roads that cross the border with the United States. The border crossing at Windsor and Detroit is the busiest in Canada, and although the 401 itself does not extend the last few kilometres into Detroit, it is the only route from Toronto to Windsor. A large portion of trade between Canada and the United States is carried on the 401.
It also serves as the principal connection to Montreal and points east. At the Quebec border, it becomes Autoroute 20.
See also: 400-Series Highway