Ontario provincial highway 401, which is normally referred to simply as Highway 401, is a freeway that extends across Southern Ontario, Canada. Known officially as the Macdonald-Cartier Freeway, it is (by far) the longest 400-Series Highway in Ontario and one of the busiest highways in the world. Together with Quebec Autoroute 20, it is the road transportation backbone of the Quebec City-Windsor Corridor, containing over half of Canada's population.

Highway 401 begins at Highway 3, 13 kilometres from the Detroit River on the outskirts of Windsor, Ontario (not at the Michigan border, as some mistakenly assume) and ends at the Quebec border, 815 kilometres away. There are 18 service station oases located along the route, allowing motorists to access services without leaving the highway. A plaque was erected at the Ivy Lea oasis, where the freeway was completed, stating that the 401 is the longest non-toll freeway under a single highway authority in North America. (The Texas section of Interstate 10 holds this record today.)

The section through Toronto (formerly referred to as Toronto By-pass) 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 separate express and collector lanes, similar to the express/local set-up of the Dan Ryan Expressway in Chicago. By 1967 the highway was widened from four lanes to 12 or more through Toronto and Mississauga. Today, the stretch of highway that passes through the Greater Toronto Area ranges from 10 to 20 lanes, and the stretch between Mississauga and Brock Road in Pickering is thought to be the world's longest continuous stretch of highway having 12 or more lanes. A 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/collector 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, Highway 35/115, and Highway 416; Quebec Autoroute 20 continues the highway at its eastern end. There are no direct interchanges with U.S. Interstate highways, but Interstates 75 and 375 in Detroit, Michigan, and Interstate 81 in New York State, are each a few miles away, via Ontario highways 3, the former 3B, and 137, leading respectively to the Ambassador Bridge, the Detroit-Windsor Tunnel, and the Thousand Islands Bridge.

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. This name is found on maps and official documents, but seldom used in conversation or on road signs.

The 401 is widely considered to be North America's busiest highway, with an estimated Annual Average Daily Traffic (AADT) of over 425,000 in 2004, near the interchange with Highway 400. Due to its triple use as the main trade, commuting and recreational corridor in Ontario, the AADT rises to well beyond the 500,000 level on some days. The just-in-time inventory systems of the highly integrated auto industry in Michigan and Ontario have made the highway the busiest truck route in North America. The 401 also includes the continent's busiest multi-structure bridge at Hogg's Hollow in Toronto (four structures for the highway's four roadway beds).

To manage traffic, the Ontario Ministry of Transportation (MTO) installed the COMPASS Freeway Traffic Management System, the most advanced of its kind in the world when it was deployed in 1991. Using a combination of CCTV cameras, vehicle detection loops and LED changeable message signs, the MTO Traffic Operations Centre can obtain a real-time assessment of traffic conditions and alert highway travellers of collisions and construction.

The 401 is one of the most important highways in Canada, as it connects the populous Southern Ontario region with Quebec and Michigan, while also connecting to most other major highways in the province. The highway also serves as the principal connection to Montreal and points east, becoming Autoroute 20 at the Quebec border. The border crossing at Windsor and Detroit is the busiest trade crossing in the world, and although the 401 itself does not extend the last few kilometres into Detroit, it is the only route from Toronto to Windsor. Some 40 percent of Canada-US trade travels the highway, which is one-third of Canada's foreign trade, and 4 percent of the all US foreign trade.

Cities along the route of the highway include Windsor, Chatham, London, Woodstock, Kitchener, Cambridge, Guelph, Mississauga, Toronto, Oshawa, Cobourg, Belleville, Kingston, Brockville, and Cornwall.

• There are many interchanges on Highway 401. Click here to view the list.

• 400-Series Highway
• List of Ontario provincial highways

• Ontario Ministry of Transportation
• Live COMPASS 401 Traffic Cameras through the Greater Toronto Area
• Photographs of the Macdonald-Cartier Freeway (Highway 401)
• Highway 401 Images from Windsor to London