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Macdonald-Cartier Freeway
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Route information
Existed	1947 (received official number designation in 1952)–present
Location
Major cities	Windsor, London, Kitchener, Toronto, Kingston, Cornwall

Highway 401 (named the Macdonald-Cartier Freeway and also "The King's Highway No. 401") is a freeway that extends across Southern Ontario. It is the longest 400-Series Highway in Ontario and one of the busiest 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. (Today the Texas section of Interstate 10 holds this record.)

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.^[1] 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, respectively.

The 401 is widely considered to be the world's busiest highway^{[citation needed]}, with an estimated Annual Average Daily Traffic (AADT) of over 425,000 in 2004, between the Weston Road and Highway 400 interchanges in Toronto. This surpasses the Santa Monica Freeway in Los Angeles, and several Interstate freeways in Houston, Texas^{[citation needed]}. Due to its triple use as the main trade, commuting and recreational corridor in Ontario, 24-hour traffic volumes can exceed 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 the world. Highway 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).

The 401 is the most important highway 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, including New England, 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 physically extend the last few kilometres into Detroit, it is the only route from Toronto to Windsor and on to Interstate 75. A future expansion of the Windsor-Detroit border crossing, which will include a freeway bypass of the existing Highway 3, may result in Highway 401 having a direct freeway link to the border. Some 40 % of Canada-US trade travels the highway, which is one-third of Canada's foreign trade, and 4 % of all US foreign trade. However, it is not part of the Ontario section of the Trans-Canada Highway.

Today the stretch of Highway 401 that passes through the Greater Toronto Area ranges from 6 to 22 lanes.

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.

Highway 401 has a separate collector-express system in Mississauga between the junctions with Highways 403/410 and Highway 427. This was completed in 1986, concurrent with the expansion of the 401-403 interchange and the addition of two express flyover ramps and includes an eastbound collector-express transfer known as the "Tunnel." At 22 lanes wide this is the widest section of Highway 401, although at the present only 9 lanes are designated for 401 through traffic (as express lanes) while the collector lanes serve as direct connections or ramp extensions from 403/410 to 427. The existing 401/427 interchange remains a bottleneck to possible east expansion of this configuration that would link it up with the Islington-Brock collector-express system. Another choke point is at the 403/410 interchange since the 401 narrows down to 6 lanes west of that junction. However, that junction has been designed with westward expansion of the collector-express system in mind; several prerequisite projects to accommodate widening from 403/410 to Mississauga Road were underway in the late 1990s, including a new interchange at Mavis Road and the reconstruction of the Mississauga Road and Derry Road overpasses.

A plan is currently underway by the Ministry of Transportation to widen the highway to at least six lanes for its length from Windsor to the Quebec border. In the 1950s, Highway 401 had initially been constructed as a 4 lane divided highway but the narrow grass median has since proven insufficient in preventing cross-directional collisions. In 1999, the Windsor-London stretch of Highway 401 was infamously known as "Carnage Alley" after a slew of fatal accidents, including an 87 vehicle pileup on Labour Day during thick fog that claimed seven lives. Much of the upgrade work will involve replacing the median with an "Ontario tall-wall" concrete barrier and an extra lane per direction.

The first sections to see these upgrades are between Windsor and Tilbury, around the City of Kingston, the London area, from Port Hope, Ontario to Cobourg, and west from Cambridge towards Woodstock. This will be followed later by staged upgrades between Trenton and Belleville, and eventually the remaining sections between Cobourg and Kingston. Other sections, namely between London and Chatham/Tilbury, and from the Kingston area, east to the Quebec border, remain beyond the 30-year planning horizon.

On November 14, 2005, the joint Canadian-American committee studying the options for expanding the Windsor - Detroit border crossing announced that its preferred option was to directly extend Highway 401 westward, using a new bridge or tunnel to cross the Detroit River and interchange with Interstate 75 somewhere between the existing Ambassador Bridge span and Wyandotte. The exact route of this new highway connection has not yet been determined.^[2]

The Ontario government is also planning to widen the Mississauga stretch from 6 lanes to 12 lanes from the 403-410 interchange to the Credit River, and 10 lanes to Mississauga Road. The Derry Road underpass and Mississauga Road overpass have been widened to accommodate this, while a new interchange has been added at Mavis Road. The main obstacles to Highway 401 expansion are the aging Creditview Road, McLaughlin Road, and Hurontario Street overpasses which will be demolished and replaced as part of the reconstruction.

The busy Wellington Road interchange at London will be completely redesigned and rebuilt to a parclo interchange. Construction began in 2006.

In the late Fall of 2005, the Ministry of Transportation began an upgrade project of the current 4-lane section of Highway 401 from west of Oxford Road 3 to west of Regional Municipality of Waterloo|Waterloo Road 97 in the counties of Oxford and Regional Municipality of Waterloo. This is currently the last 4-laned section between London and Toronto, but is being upgraded to 6 lanes plus an Ontario tall-wall barrier in the median.

Aside from the addition of one lane per direction and a tall-wall barrier, two emergency onramp accesses to be used only during highway closures and other such incidents have been constructed. These emergency accesses are the first of their kind on an Ontario freeway, and have been constructed in the vicinity of both Oxford Road 3, and Trussler Road. [1]

Other emergency accesses have been built in Essex County where the road was recently widened from 4 to 6 lanes, but these are gravel, and all are alongside overpasses, for use by ambulances, firetrucks, and police cars in emergencies only.

To manage traffic, the Ontario Ministry of Transportation (MTO) installed the Freeway Management System - COMPASS, the most advanced of its kind in the world when it was deployed in 1991 by Delcan Corporation. 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 system currently stretches from Highway 403, Mississauga to Harwood Avenue, Ajax, and is likely to be extended westward and eastward as Highway 401 is reconstructed and upgraded.

Highway 401 has found expression in contemporary music.

• Kingston, Ontario, rock band The Tragically Hip's 1994 album Day for Night features Titanic Terrarium, whose lyrics include:

"...we don't F--- with the 401. It's bigger than us, or larger than we bargained."

• Winnipeg singer-songwriter James Keelaghan referred to The 401 in his 1995 song Get to You:

"There's a rig spun out on 401/Closed it down to cars and vans."

• And Stompin' Tom Connors has a number of songs dealing with The 401, including Bud the Spud and Canada Day up Canada Way

• Highest Volume: 607,800 AADT from Weston Road (Exit 357) to Highway 400 (Exit 359) compared to 510,000 in 2003.
• Lowest Volume: 15,000 AADT from Highway 3B (Exit 13) to Highway 3, Bridge to U.S.A.

From west to east, highway 401 signage uses the following control cities: Windsor, London, Toronto, Kingston, Cornwall, Montreal. Within the City of Toronto (between Highway 427 and Meadowvale Road), control cities are not used.

Main article: List of exits on Highway 401 (Ontario)

The service centres are located at the following points on Highway 401 and contain the following services:

• 400-Series Highways
• 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 at OntHighways.com
• Highway 401 - the Story
• Truck Stops Within Ontario
• TravelCenters of America (operators of the Woodstock/Ingersoll Service Centers
• Highway 401 History