The King's Highway No. 401 (named the Macdonald-Cartier Freeway) is a freeway that extends across Southern Ontario, Canada. It is 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 along which over half of Canada's population lives. Colloquially, the road is known simply as "The 401," spoken as "four-oh-one".

The first segment of the highway (originally designated Highway 2A) from Highland Creek (Scarborough) to Oshawa was begun in 1938 to bypass a congested section of nearby Highway 2. Much of the grading and a number of structures were completed before the onset of the Second World War, at which time most of the construction work was shut down. This section was finally completed and opened to traffic in 1947.

During the war years, the provincial government undertook a number of surveys and studies to determine the most desirable route for the new limited-access highway.

The highway was redesignated Highway 401 in 1952. The last segment, between Gananoque and Brockville, was completed in 1968 to bypass a section along the St. Lawrence River that included at-grade intersections and private entrances, now called the Thousand Islands Parkway. The last at-grade intersection on the present alignment was at Joyceville Road, east of Kingston, which was converted to an interchange with the final 1968 construction.

In 1965, Ontario Premier John Robarts designated Highway 401 the Macdonald-Cartier Freeway, in honour of Sir John A. Macdonald and Sir George-Étienne Cartier, the Province of Canada's most important Fathers of Confederation, and in recognition of Canada's upcoming Centennial Celebrations in 1967. This name is found on maps and official documents, but seldom used in conversation or on modern road signs. A 1961 proposal to name the highway the "Leslie M. Frost Thruway" after the recently retired Premier of Ontario was approved by the provincial cabinet, but was not carried out.

Highway 401 begins at Highway 3, 13 kilometres (8.08 miles) from the Detroit River on the outskirts of Windsor (not at the Michigan border as some mistakenly assume) and ends at the Quebec border, 815 kilometers (506.42 miles) away. There are 18 rest areas or service centres (oases) located along the route, allowing motorists to access services without leaving the highway. A plaque was erected at the Mallorytown oasis, located on the last section of the freeway to be completed, stating that the 401 was the longest non-toll freeway under a single highway authority in North America. This record was later superseded - the Texas section of Interstate 10 holds this record today.


Cities along the route of the highway include Windsor, Chatham, London, Ingersoll, Woodstock, Kitchener, Cambridge, Guelph, Milton, Mississauga, Toronto, Pickering, Ajax, Whitby, Oshawa, Cobourg, Trenton, Belleville, Kingston, Brockville, and Cornwall. The control cities are listed below.
Major freeway junctions are located at these roads and highways: 402, Highbury Avenue (London), 403, 8, 6, 407, 410, 427, 400, Allen Road (Toronto), 404 and Don Valley Parkway (Toronto), 35 and 115, 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 short distance away, via Ontario highways 3, the former 3B, and 137, leading to the Ambassador Bridge, the Detroit-Windsor Tunnel, and the Thousand Islands Bridge.

The section that now runs through Toronto was a rural roadway that was entirely outside of the Toronto city limits when first opened, and was originally referred to as the Toronto Bypass. The new freeway attracted development all along its length. As the city's suburbs grew, it quickly became an urban commuter road, rather than a long-distance bypass route as was originally planned, leading to extensive traffic jams. This was a problem/opportunity solved and built upon 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 Highway 401 was widened from four lanes to 12 or more through Toronto from Islington Avenue to Warden Avenue. In the early 1970s the collector express lanes were extended to Neilson Road and eastward to Brock Road by 1997. An alternative plan to the collector-express setup was to construct a bypass of Highway 401. This project was revived in 1987 and opened in 1997 as the toll road Highway 407 ETR to parallel Highway 401 in the Greater Toronto Area.

The main collector-express setup with a minimum 12 lane cross-section currently stretches from Islington Avenue to Brock Road. Highway 409 which branches off from Highway 401 just west of Islington to Toronto Pearson International Airport has been unsuccessful in diverting traffic volumes; the 10 lane segment between Islington and Highway 427 is highly congested as a result. The existing 401-427 interchange is considered one of the worst bottlenecks since it only allows 8 lanes of traffic (4 in each direction) to pass through the interchange. It initially only allowed 6 lanes of through traffic until a widening project was completed in 2005; a major reconstruction of the junction is needed to meet current demands estimated at 5-6 lanes.