# Ontario Highway 401

King's Highway 401, commonly referred to as Highway 401 and also known by its official name as the Macdonald–Cartier Freeway or colloquially referred to as the four-oh-one, is a 400-series highway in the Canadian province of Ontario. It is an 828-kilometre-long (514 mi) freeway, travelling from Windsor in the west to the Ontario–Quebec border in the east (west of Montreal). The part of Highway 401 that passes through Toronto is North America's busiest highway, and one of the widest in Canada. Together with Autoroute 20 (A-20), it forms the road transportation backbone of the Quebec City–Windsor Corridor, along which over half of Canada's population resides. It is also a Core Route in Canada's National Highway System. The route is maintained by the Ministry of Transportation of Ontario (MTO) and patrolled by the Ontario Provincial Police (OPP). The speed limit is 100 km/h (62 mph) throughout the majority of its length, with the remaining exceptions being the posted 80-kilometre-per-hour (50 mph) limit westbound in Windsor, in most construction zones, and the 110-kilometre-per-hour (68 mph) speed limit on the 40-kilometre (25 mi) stretch between Windsor and Tilbury that was raised on April 22, 2022, the 7-kilometre (4.3 mi) extension east of the aforementioned, the 35-kilometre (22 mi) stretch between Highway 35 / 115 and Cobourg, the 44-kilometre (27 mi) stretch between Colborne and Belleville, the 66-kilometre (41 mi) stretch between Belleville and Kingston, and the 107-kilometre-long (66 mi) stretch between Highway 16 and the east end of the freeway that were raised on July 12, 2024.
By the end of 1952, three individual highways were numbered "Highway 401": the partially completed Toronto Bypass between Weston Road and Highway 11 (Yonge Street); Highway 2A between West Hill and Newcastle; and the Scenic Highway between Gananoque and Brockville, now known as the Thousand Islands Parkway. These three sections of highway were 11.8, 54.7, and 41.2 km (7.3, 34.0, and 25.6 mi), respectively. In 1964, the route became fully navigable from Windsor to the Ontario–Quebec border. In January 1965, it was given a second designation, the Macdonald–Cartier Freeway, in honour of two Fathers of Confederation. At the end of 1968, the Gananoque–Brockville section was bypassed and the final intersection grade-separated near Kingston, making Highway 401 a freeway for its entire 817.9-kilometre (508.2 mi) length. In August 2007, a portion of the highway between Trenton and Toronto has been designated as the Highway of Heroes, as the route is travelled by funeral convoys for fallen Canadian Forces personnel from CFB Trenton to the coroner's office in Toronto.
Highway 401 previously ended at Highway 3 (Talbot Road) upon entering Windsor. In 2011, construction began on a westward extension called the Rt. Hon. Herb Gray Parkway (formerly Windsor-Essex Parkway). This extension runs parallel to Highway 3 (Talbot Road and Huron Church Road) between the former end of the freeway and the E. C. Row Expressway, at which point the extension turns and runs alongside the E.C. Row towards the future Gordie Howe International Bridge. An 8-kilometre (5.0 mi) section of the parkway, east of the E. C. Row interchange, opened on June 28, 2015, with the remaining section completed and opened on November 21. The widening of the highway between Highway/Regional Road 8 in Kitchener to Townline Road in Cambridge to at least ten lanes was completed by December 22, 2023. There are plans underway to widen the remaining four-lane sections between Windsor and London to six lanes and to widen the route between Cambridge and Milton as well as through Oshawa. The expansive twelve-plus-lane collector–express system through Pickering and Toronto, and partially across Mississauga, was extended west to Milton in December 2022.


## Route description
Highway 401 extends across Southwestern, Central, and Eastern Ontario. In anticipation of the future expansion of the highway, the transportation ministry purchased a 91.4-metre-wide (300 ft) right-of-way along the entire length. Generally, the highway occupies only a portion of this allotment.
It is one of the world's busiest highways; a 2019 analysis stated the annual average daily traffic (AADT) count between Renforth Drive and Highway 427 in Toronto was at 450,300,
while a second study estimates that over 500,000 vehicles travel that section on some days. This makes it North America's busiest roadway, surpassing the Santa Monica Freeway in Los Angeles and I-75 in Atlanta. The just-in-time auto parts delivery systems of the highly integrated automotive industry of Michigan and Ontario have contributed to the highway's status as the world's busiest truck route, carrying 60 percent of vehicular trade between Canada and the US.
Highway 401 also features North America's busiest multi-structure bridge at Hogg's Hollow in Toronto.
The four bridges, two for each direction with the collector and express lanes, carried an average of 360,300 vehicles daily in 2019.
The highway is one of the major backbones of a network in the Great Lakes region, connecting the populous Quebec City–Windsor corridor with Michigan, New York and central Ontario's cottage country.
It is the principal connection between Toronto and Montreal, becoming A-20 at the Ontario–Quebec border.


### Southwestern Ontario
Highway 401 does not yet extend across the Detroit River into Detroit. However, by early 2026, the Gordie Howe International Bridge will extend Highway 401 across the Canada–United States border to a connection through Delray to I-75.
At present, Highway 401 begins as a six-lane freeway in Brighton Beach (at the Canada–US border in Windsor) at the west end of the E. C. Row Expressway, sandwiched between the E. C. Row's opposing carriageways for a short distance. Highway 401 then changes to a southeast direction as it descends into a trench and runs alongside Highway 3 (Huron Church Road and Talbot Road). This below-grade section of the highway has 11 tunnels to cross underneath surface streets (including those carrying the Highway 3 routing). After passing under Howard Avenue and the ramps to Talbot Road, the highway then curves northeast and ascends back to the surface. At the split with Dougall Parkway (former Highway 3B) which links to the Detroit–Windsor tunnel, the highway turns east and exits Windsor. From here, Highway 401 mostly parallels the former route of Highway 98 from Windsor to Tilbury.

Southwestern Ontario is flat, primarily agricultural land, that takes advantage of the fertile clay soil deposited throughout the region.
The main river through the region is the Thames River, which drains the second largest watershed in southern Ontario and largely influences the land use surrounding the highway.
It parallels the route to the north between Tilbury and Woodstock.
Near Tilbury, Highway 401 loses its tall wall median barrier, then narrows to four lanes, following lot lines laid between concession roads in a plan designed to limit damage to the sensitive agricultural lands through which the highway runs.
Here the highway's flat, straight route is notorious for leading to driver inattention.
The section from Windsor to London (especially west of Tilbury) has become known for deadly car accidents and pile-ups, earning it the nickname Carnage Alley.
As the highway approaches London, Highway 402 merges in, resulting in a six-lane cross-section.
Within London, it intersects the city's two municipal expressways, Highbury Avenue and the Veterans Memorial Parkway.
The section between London and Woodstock generally parallels the former Highway 2 but lies on the south side of the Thames River. This area is not as flat, but the highway is generally straight. This part of Highway 401 often experiences heavy snowsqualls in early winter, sometimes extending as far east as Toronto. To the south of Woodstock, Highway 401 curves northeast as Highway 403 splits off at its western terminus. The freeway then meets the former Highway 2 at an interchange, reconfigured from a cloverleaf to a five-ramp parclo in the late 2000s, near the Toyota West Plant. From here, the highway heads towards Kitchener and Cambridge, substantially north of the route of the former Highway 2, which has been bypassed by Highway 403's western leg.
Heading towards Kitchener, the highway ascends as it crosses the Grand River followed by interchanges with King Street (Waterloo Regional Road 8) and Highway 8 before returning to its eastward orientation.  Between Highway/Regional Road 8 and Highway/Regional Road 24 in Cambridge, the highway was widened in 2020 to twelve lanes to accommodate the growing traffic using that segment.
Beyond Highway/Regional Road 24, the highway returns to a six-lane cross section and meanders towards Milton, passing through hills and rock cuts along the way. As it enters Milton, it also enters Halton Region, part of the Greater Toronto Area.


### Greater Toronto Area
As Highway 401 approaches the Greater Toronto Area (GTA) from the west, it rounds Rattlesnake Point (part of the ecologically protected Niagara Escarpment) to the west of Milton.
Upon entering the town, it enters the first urbanized section of the GTA, passing through two rural areas between there and Oshawa. Part of this rural gap is the western side of Toronto's Greenbelt, a zone around Toronto protected from development. After this 10-kilometre (6.2 mi) gap, the highway interchanges with the Highway 407 Express Toll Route (407 ETR). Within the GTA, the highway passes several major shopping malls including Toronto Premium Outlets, Yorkdale Shopping Centre, Scarborough Town Centre, and Pickering Town Centre.

Within Greater Toronto, three separate segments of Highway 401 employ a collector-express system, a concept inspired by the Dan Ryan Expressway in Chicago. The system divides each direction of travel into segregated collector and express lanes,
giving the highway a wide span and four carriageways. Unlike the collector lanes, which provide access to every interchange, the express lanes only provide direct access to a select few interchanges. Access between the two is provided by transfers, which are strategically placed to prevent disruptions caused by closely spaced interchanges. To avoid confusion between carriageways, blue signs are used for the collector lanes and green signs for the express lanes. The overall purpose of the collector-express system is to maximize traffic flow for both local and long-distance traffic.
From the west, the first collector-express section through the GTA is 4.6 km (2.9 mi) long and runs from James Snow Parkway to Highway 407.
The freeway briefly narrows to 10 lanes between the interchanges with Highway 407 and Winston Churchill Boulevard, then the second collector-express system begins and runs 16.7 km (10.4 mi) to Highway 427. The west end of this section initially terminated just west of Highway 410 in the early 1990s. It was extended westward in stages during the 2010s to include the interchanges with Hurontario Street, Mavis Road, and Mississauga Road; with the final extension to Winston Churchill Boulevard (and to Milton) being completed in 2022. Because Highway 403's eastern terminus feeds into the easternmost segment of the freeway's second collector-express system, this results in the Highway reaching its widest point, at 18 lanes. Running south of Toronto Pearson International Airport, the close proximity to the runways necessitates conventional light poles instead of high-mast lighting. Approaching the City of Toronto's western border, a final transfer (known as the "Airport Basketweave") allows eastbound traffic in the collector lanes to transfer to Highway 401 express lanes which continue past Highway 427 as the single eastbound Highway 401 carriageway. The Highway 401 collector lanes then become ramps to Highway 427 after a final exit at Renforth Drive and crossing into Toronto. For westbound traffic, the Highway's single westbound carriageway becomes the Highway 401 express lanes. The exit ramps from Highway 427 merge to form the start of the westbound Highway 401 collector lanes in this section. The collector lanes were originally designed to accommodate and organize various traffic movements from the Highway 403 / Highway 410 and Highway 427 interchanges along Highway 401, replacing an earlier plan that would have run Highway 403 directly to Eglinton Avenue and the never-built Richview Expressway.
East of the interchange with Highway 427, the single carriageway of Highway 401 curves northeast and follows a hydro corridor to the east end of Highway 409. The space constraints of the existing flyovers at the 401/427 interchange also limit the width of Highway 401 through this junction to eight lanes (widened from the original six), making it a traffic bottleneck. Due to ramps from Highway 427's southernmost segment (between Highway 401 and the QEW) feeding in, this 5-kilometre (3.1 mi) segment of Highway 401 between Highways 427 and 409 is six lanes per direction (five lanes through the interchange with Dixon Road and Martin Grove Road), with no express/collector split. Highways 409 is a short freeway used mainly as a spur route for traffic travelling to the airport or Highway 427 northbound from Highway 401 westbound (and vice versa), as these route movements are not accommodated at the junction between Highways 401 and 427.
At the terminus of Highway 409 where the ramps merge into Highway 401, Highway 401 returns to its east–west route through Toronto. This is also the west end of the third and longest express-collector segment (43.7 km (27.2 mi)), which crosses the rest of Toronto to Brock Road in Pickering in the east. An average of 442,900 vehicles pass between Weston Road and Highway 400 (Toronto–Barrie Highway) per day as of 2008, making this segment of Highway 401 in Toronto the busiest freeway in the world. In spite of this congestion, it is the primary commuting route in Toronto, and over 50 percent of vehicles bound for downtown Toronto use the highway. The four-way junction with Highway 400 has flyover ramps that favor traffic north of that interchange, as south of that interchange Highway 400 has an exit to Jane Street before the route continues further south as the municipal expressway Black Creek Drive. Between the interchanges with Highway 400 and Keele Street is a set of transfer ramps between the express and collector lanes nicknamed "The Basketweave", as each direction has a pair of ramps that cross over and under each other.

Passing between Downsview Airport (decommissioned as of April 2024) and Yorkdale Shopping Centre, access for Highway 401 motorists west of Dufferin Street is provided by the latter's partial interchange. Access to Yorkdale for motorists to/from Highway 401 east of Allen Road (a short municipal expressway) is provided by the latter's complicated turbine/combination interchange. Between Avenue Road and Yonge Street, Highway 401 crosses North America's busiest multi-structure bridge at Hogg's Hollow.
The four bridges, two for each direction with the collector and express lanes, carried an average of 360,300 vehicles daily in 2019. After passing south of North York City Centre and Bayview Village, the freeway descends as it approaches an interchange with Leslie Street, which provides an exit/entry for North York General Hospital and Oriole GO Station. East of the Leslie Street Interchange, Highway 401 ascends as it crosses the Don River and approaches the major junction with Highway 404 and the Don Valley Parkway (DVP).

Between Birchmount Road and Midland Avenue, Highway 401 is elevated on a berm as it crosses three surface streets and two railway lines. Progressing eastward in Scarborough, the Highway 401 continues through mostly residential areas and Scarborough City Centre including the shopping mall. At the northwest corner of the interchange with McCowan Road is a Bell Media/CTV studio complex and CTV Television Network's studio office, 9 Channel Nine Court. Highway 401 eventually reaches Toronto's eastern edge where at Meadowvale Road, it turns southeast briefly before crossing Kingston Road as it swings northeast as Highway 2A (downloaded from the province to the municipality in 1998) merges into it, followed immediately by an interchange with Port Union Road/Sheppard Avenue, then crossing the Rouge Valley into Pickering and Durham Region.
West of Pickering, Highway 401 again meets former Highway 2, which thereafter parallels it to the Ontario–Quebec border. As the highway approaches an interchange with Brock Road (Durham Regional Road 1), the collector and express lanes converge, narrowing the 14-lane freeway to 10, divided only at the centre.
It remains this width as it passes into Ajax,
before narrowing back to six lanes at Salem Road (Durham Regional Road 41).
Planned expansions east of Salem to improve flow leading into the Highway 412 (West Durham Link) and Lakeridge Road (Lake Ridge Road) (Durham Regional Road 23) interchanges will see the highway widened to ten lanes as far as Brock Street in Whitby, where the existing interchange will be reconfigured.
East of Ajax, the highway passes through the second 3.5-kilometre (2.2 mi) rural gap, and enters Whitby. The stretch of Highway 401 through Whitby and Oshawa features several structures completed during the initial construction of the highway in the 1940s. Several of these structures are to be demolished, either due to their age, or to prepare for the planned widening of Highway 401 through this area.
A former Canadian National Railway overpass, which was fenced off but commonly used by pedestrians during Highway of Heroes repatriations, was demolished on the night of June 11, 2011. A second structure in Bowmanville was demolished during two overnight closures on July 9 and 16.
At Harmony Road (Durham Regional Road 33), the suburban surroundings quickly transition to agricultural land. Travelling alongside another hydro corridor, the highway curves around the south side of Bowmanville and travels towards Highway 35 and Highway 115.


### Eastern Ontario
From east of Highway 35 and Highway 115 to Cobourg, Highway 401 passes through a mix of agricultural land and forests, maintaining a straight course.
After exiting Greater Toronto, the freeway passes through the north end of the towns of Port Hope and Cobourg with two interchanges each. Just east of Cobourg, the highway narrows to four lanes and the terrain becomes undulating, with the highway routed around hills and through valleys along the shores of Lake Ontario.
At Trenton, the highway crosses the Trent Canal and returns to an agricultural setting. It then crosses the Moira River as it goes through Belleville, before passing through Greater Napanee, then heading eastward to Kingston. The Kingston portion of the highway, originally named the Kingston-Bypass, was one of the first sections of the highway to be completed; it is now mostly three lanes each way.

East of Kingston, the highway continues through a predominantly agricultural area alongside the St. Lawrence River to Gananoque, where it splits with the Thousand Islands Parkway, one of the original sections of the highway designated in 1952. The highway runs parallel to the parkway several kilometres inland from the river. The Canadian Shield, an ancient geological formation, appears through this heavily forested section of the highway. Highway 401 rejoins the Thousand Islands Parkway immediately southwest of Brockville, now heading northeast.
The remainder of the highway runs parallel to the former Highway 2 along the shore of the St. Lawrence River within the St. Lawrence Valley. Northeast of Brockville is the interchange with Highway 416, which heads north to Ottawa. The freeway passes through Johnstown, Cardinal, Iroquois, Morrisburg, Ingleside, Cornwall, and the last towns of Lancaster and South Lancaster. At the Ontario–Quebec Border, Highway 401 becomes Autoroute 20 and continues to Montreal.


### Traffic volume
The MTO publishes yearly traffic volume data for provincial highways, expressed as an average daily vehicle count over the span of a year (AADT).
The table below compares the AADT at several locations along Highway 401 using data from 1969, 1988, 2008, 2016, and 2021.


## History
### Predecessors
Highway 401's history predates its designation by over two decades. As automobile use in southern Ontario grew in the early 20th century, road design and construction advanced significantly. Following frequent erosion of Lake Shore Road, then macadamized,
a concrete road known as the Toronto–Hamilton Highway was proposed in January 1914. Construction began on November 8 of that year, following the onset of World War I.
The highway was designed to run along the lake shore, instead of Dundas Street to the north, because the numerous hills encountered along Dundas would have increased costs without improving accessibility. Middle Road, a dirt lane named because of its position between the two, was not considered since Lake Shore and Dundas were both overcrowded and in need of serious repairs.
The road was formally opened on November 24, 1917, 5.5 m (18 ft) wide and nearly 64 km (40 mi) long. It was the first concrete road in Ontario, as well as one of the longest stretches of concrete road between two cities in the world.
Over the next decade, vehicle usage increased substantially, and by 1920, Lakeshore Road was again congested, particularly during weekends.
In response, the Department of Highways examined improving another road between Toronto and Hamilton. The road was to be more than twice the width of Lakeshore Road at 12 m (39 ft) and would carry two lanes of traffic in either direction.
Construction on what was then known as the Queen Street Extension west of Toronto began in early 1931.
Before the highway could be completed, Thomas McQuesten was appointed the new minister of the Department of Highways, with Robert Melville Smith as deputy minister, following the 1934 provincial elections.
Smith, inspired by the German autobahns—new "dual-lane divided highways"—modified the design for Ontario roads,
and McQuesten ordered the Middle Road be converted into this new form of highway.
A 40 m (130 ft) right-of-way was purchased along the Middle Road and construction began to convert the existing sections to a divided highway. Work also began on Canada's first interchange at Highway 10.

Beginning in 1935, McQuesten applied the concept of a dual-highway to several projects along Highway 2, including along Kingston Road in Scarborough Township.
When widening in Scarborough reached the Highland Creek ravine in 1936, the Department of Highways began construction on a new bridge over the large valley, bypassing the former alignment around West Hill.
From here the highway was constructed on a new alignment to Oshawa, avoiding construction on the congested Highway 2. As grading and bridge construction neared completion on the new highway between West Hill and Oshawa in September 1939, World War II broke out and gradually tax revenues were re-allocated from highway construction to the war effort.
At the same time, between September 6 and 8, 1939, the Ontario Good Roads Association Conference was held at Bigwin Inn, near Huntsville,
drawing highway engineers from across North America to discuss the new concept of "Dual Highways". On the first day of the convention, McQuesten announced his vision of the freeway: an uninterrupted drive through the scenic regions of Ontario, discouraging local business and local traffic from accessing the highway except at infrequent controlled-access points.
It was announced in the days thereafter this concept would be applied to a new "trans-provincial expressway", running from Windsor to the Ontario–Quebec border.
Highway engineers evaluated factors such as grading, curve radius, and the narrow median used along the Middle Road—which was inaugurated on August 23, 1940, as the Queen Elizabeth Way (QEW)—and began to plan the course of a new dual highway mostly parallel to Highway 2, with precedence given to areas most hampered by congestion. Unlike the QEW, this highway would not be built along an existing road, but rather on a new right-of-way, avoiding the need to provide access to properties.
Along with immense improvements to machinery and construction techniques over its six-year course, the war provided planners an opportunity to conduct a survey of 375,000 drivers, asking them about their preferred route to travel to their destination. Using this information, a course was plotted from Windsor to Quebec, bypassing all towns along the way.
Highway 2S (S for Scenic) was the first completed section of new roadway. Built to connect with the Thousand Islands Bridge at Ivy Lea and opened as a gravel road in late 1941 or early 1942,
the road followed the shore of the Saint Lawrence River and connected with the western end of the twinned Highway 2 near Brockville.
In addition, the highway between Highland Creek and Oshawa was opened as a gravel-surfaced road in May 1942.
Following the war, construction resumed on roadways throughout Ontario. The expressway between Highland Creek and Oshawa was completed in December 1947 and designated as Highway 2A, while other sections languished. The Toronto–Barrie Highway was the primary focus of the Department of Highways at the time, and the onset of the Korean War in 1950 stalled construction again. Despite the delays, highway minister George Doucett officially announced the plans for construction of the new trans-provincial expressway that year, with the Toronto to Oshawa expressway serving as a model for the design. Work on the most important link, the Toronto Bypass, began in 1951, but it would not open with that name.


### Assumption
In July 1952 (possibly July 1, the same day Highway 400 was numbered),
the Highland Creek to Oshawa expressway (Highway 2A) and Highway 2S were designated Controlled-Access Highway No 401, a move scorned by one critic because of the lack of thought given to the numbered name.
Construction was completed for several sections of the Toronto Bypass: between Highway 400 and Dufferin Street in August, west to Weston Road in September, east to Bathurst Street in October and finally to Yonge Street in December. A 3.4 km (2.1 mi) stub of Highway 2A which was not incorporated into Highway 401 remained a provincial route as a connector to Kingston Road.
Extensions east and west began in 1953; the eastern extension to Bayview Avenue opened in April 1955, but the western extension was delayed by the damage caused by Hurricane Hazel on October 15, 1954, which nearly destroyed the new bridge over the Humber River. The reconstruction would take until July 8, 1955,
and the highway was opened between Weston and Highway 27 in September 1955.

The entire bypass, including the widening of Highway 27 into an expressway south of Highway 401,
was completed in August 1956.
Upon its opening, the bypass was described by one reporter as "a motorist's dream" providing "some of the most soothing scenery in the Metropolitan area". The reporter continued, with regard to the eastern section through Scarborough, that it "winds smoothly through pastures across streams and rivers, and beside green thickets. It seems a long way from the big city."
By 1959 however, the bypass was a lineup of cars, as 85,000 drivers crowded the roadway, designed to handle a maximum of 48,000 vehicles, on a daily basis.
Motorists found the new road to be a convenient way of travelling across Toronto; this convenience helped influence the suburban shift in the city and continues to be a driving force of urban sprawl today.
Meanwhile, beyond Toronto, the highway was being built in a patchwork fashion, focusing on congested areas first. Construction west from Highway 27 began in late 1954,
as did the Kingston Bypass in Eastern Ontario.
Work began to connect the latter with the Scenic Highway in 1955.
After the 1954 New York State Thruway opened from Buffalo to New York City, Michigan officials encouraged Ontario to bypass Highway 3 as the most direct path from Detroit to Buffalo.
By 1956, construction had begun on a segment between Highway 4 in London and Highway 2 in Woodstock, as well as on the section between Windsor and Tilbury.

In 1958, a section bypassing Morrisburg was opened to accommodate traffic displaced from a portion of Highway 2 through The Lost Villages of the Saint Lawrence Seaway.
By the end of 1960, the Toronto section of the highway was extended both eastwards and westwards: first east from Newcastle to Port Hope on June 30; then later west from Highway 25 in Milton to Highway 8 south of Kitchener on November 17.
By mid-1961, the section between Brighton and Marysville had opened. The gap to the east, from Highway 28 in Port Hope to Highway 30 in Brighton was opened on July 20 of that year.
The gap between Woodstock and Kitchener was completed on November 9, 1961, while the gap between Tilbury and London was completed two lanes at a time; the westbound lanes on October 22, 1963, the eastbound on July 20, 1965. The gap between Marysville and Kingston was opened by 1962.
The final sections, from west of Cornwall to Lancaster, were opened between 1962 and 1964;
two lanes opened to Lancaster on September 11, 1962, but the other two were not completed until July 31, 1964. The last segment, to the Ontario–Quebec border, was opened on November 10, 1964.
As originally envisioned by McQuesten, Highway 401 had been routed along the Thousand Islands Parkway since 1952.
However, by then numerous properties and a tourist industry were established, which had not been present when the parkway originally opened in 1938. James Auld, MPP for Leeds and the Minister of Tourism and Information, joined local residents to persuade the DHO to construct an inland bypass. The DHO agreed, stating that it would cost less to build a new freeway than to upgrade the parkway. Construction of the Thousand Islands Bypass began in 1965, with work proceeding east from Gananoque. The Thousand Islands Parkway was the final two-lane segment of Highway 401. 
A portion was opened on September 1, 1967, from Gananoque to Highway 137, which was itself built south to the parkway at the same time. The Highway 401 designation was applied along this new route, while the bypassed portion of the parkway was redesignated as Highway 2S.
Despite the expected influx of traffic from the United States for Expo 67 in Montreal, the DHO opted to build the portion east of Ivy Lea after the centennial celebrations.
Finally, on October 11, 1968, the Thousand Islands Bypass opened. This final piece was commemorated with a plaque to signify the completion of Highway 401.


### Expansion
In Toronto, engineers and surveyors were examining the four-lane bypass, while planners set about designing a way to handle the commuter highway. In 1963, transportation minister Charles MacNaughton announced the widening of Highway 401 in Toronto from four to a minimum of 12 lanes between Islington Avenue and Markham Road. The design was taken from the Dan Ryan Expressway in Chicago, which was widened into a similar configuration around the same time. Construction began immediately. While the plan initially called for construction to end in 1967, it continued for nearly a decade. At least four lanes were always open during the large reconstruction project, which included complex new interchanges at Highway 27, Highway 400, the planned Spadina Expressway and the Don Valley Parkway. The system was completed in 1972, along with the Highway 27 (renamed Highway 427) bypass between the QEW and Pearson Airport. Most of the interchanges in Toronto were reconstructed as partial cloverleafs and a continuous lighting system was installed.
On January 11, 1965, at the dinner celebration of Sir John A. Macdonald's 150th birthday, the Premier of Ontario John Robarts designated Highway 401 the Macdonald–Cartier Freeway to honour Macdonald and George-Étienne Cartier, two of Canada's Fathers of Confederation.
Unlike other names later applied to the highway, the Macdonald–Cartier Freeway designation covers the entire length of Highway 401. Signs designating the freeway and shields with the letters 'M-C' were installed, but these had been removed by 1997.
In 2003, 38 years after Robarts' naming of the highway, a Member of Provincial Parliament attempted to get the Macdonald-Cartier Freeway highway name enshrined into law; the bill only passed first reading and was not enacted.
In the 1970s, Highway 401 was widened to six lanes in Durham. Between 1977 and 1982, Highway 401 was widened from four to six lanes between Hurontario Street (Highway 10) to Highway 25, with the Jersey median barrier making its debut in Ontario in that segment.

Plans had been completely modified to connect the under-construction Mississauga leg of Highway 403 from QEW to Highway 401 at the new Highway 410 interchange.
For Highway 403, the Highway 410 interchange was a better connection point than the original suggested terminus at the Highway 401-427 interchange, but would also require the widening of Highway 401 between Highway 403 and Highway 427 from eight lanes to an eighteen-lane collector-express system. Plans were submitted and approved in December 1977 by Mississauga city council, and construction began. The first section of Highway 403 between Cawthra Road and Highway 401 was opened August 18, 1980; the original connecting ramps were built on the outer perimeter of the interchange to serve what would later be designated as the collector lanes of both routes, leaving sufficient right-of-way between for future ramps linking the express lanes of both routes.
The 1980s saw more sections of Highway 401 widened. Most significant was the new collector-express system between Highway 403 / 410 and Highway 427. This including a new set of flyover ramps from the express lanes to Highway 403 which opened in 1984, while a basketweave transfer between the eastbound collector and express lanes near Pearson Airport was completed in mid-1985. At this time, the Highway 401 express lanes merged/diverged into/from the collector lanes east of Tomken Road which resulted in a temporary left-handed exit/entry for the ramps to/from the Highway 403 express lanes.
After the Kennedy Road overpass was replaced, cast-in-place post-tensioned concrete flyover ramps were constructed from 1988 to late 1990 to link up Highway 401 and Highway 410, notably the 11-span flyover ramp from Highway 401 eastbound to Highway 410 northbound which remains the longest in the Greater Toronto Area, while the Highway 410 southbound to Highway 401 eastbound flyover replaced a loop ramp. In the fall of 1991, alongside the widening of Highway 410 into a full freeway, construction began on the connecting ramps between Highway 403 and Highway 410, made possible by the removal of the loop ramp (see above) which pass under the existing bridge structures carrying Highway 401 which would soon be designated as the collector lanes. In 1993, new overpasses crossing Tomken Road and the connecting ramps between Highway 403 and Highway 410 were constructed for the Highway 401 express lanes, extending Highway 401's collector-express system from east of Tomken Road to just east of Kennedy Road.

Plans were made to extend the eastern system from Neilson Road to Brock Road in Pickering in the late 1980s,
but took over a decade to reach fruition by 1997.
This was followed shortly thereafter by the widening of the highway through Ajax and a new interchange at Pickering Beach Road (renamed Salem Road) and Stevenson Road.
The 1990s also saw the first step in widening the highway from Toronto to London, by replacing the grass median with the addition of a third traffic lane per direction separated by a tall-wall concrete median barrier. The segment from London to Woodstock received this upgrade first, with the expansion shortly east of the split for Highway 403's newly constructed western segment.
A project in the mid-1990s brought the highway up to a minimum of six lanes between Highway 8 in Kitchener and Highway 35 / 115 in Newcastle.
Other projects prepared sections for eventual widening.
In its 2007 plan for southern Ontario, the MTO announced long-term plans to create high-occupancy vehicle (HOV) lanes from Mississauga Road west to Milton;
by 2011 these plans had been expanded in scope to as far west as Hespeler Road in Cambridge. 

From the mid-1990s onward, several prerequisite projects were undertaken for the future widening of Highway 401 throughout Milton and Mississauga, although that section of the freeway would retain its six-lane cross section for at least the next decade. This included the new interchange with the upcoming Highway 407 ETR (with a temporary diversion for the eastbound lanes to facilitate construction of the post-tensioned flyovers), a new underpass structure to accommodate Derry Road which was widened from two to six lanes (1996–97), the addition of an interchange with the Mavis Road extension in 1999 whose overpass was designed to be lengthened with two outer spans, and replacing the Mississauga Road overpass which began on February 15, 2002 (in a cost-sharing agreement between the province and Peel Region) and completed in 2003. More recent work included the replacement of the McLaughlin Road overpass in 2008.
Starting on August 19, 2009, Highway 401's collector-express lanes in Mississauga have been extended westward beyond its initial terminus at Highway 410.  The widening of this 7 km segment to the Credit River necessitated the replacement of several existing overpasses, including those of Hurontario Street (2013) and Second Line West (2016), with the Second Line West crossing rebuilt as a pedestrian/cyclist bridge since most vehicular traffic was already rerouted to the Mavis Road extension. Tying into this extension of Highway 401's collector-express system, the missing ramps from Highway 401 eastbound to Highway 403 westbound and the opposite movement were completed in 2018 which also provided direct access to Cawthra Road, making the Highway 401-403-410 junction a full four-way interchange.
The first phase of this expansion to west of Hurontario Street, a distance of 2.8 km (1.7 mi), opened in 2013,
while the second phase to the Credit River was completed in 2020.
In 2019, an announcement was made for expanding the freeway from the Credit River to Regional Road 25 in Milton to a minimum of 10 lanes, including HOV lanes. This project included extending the existing 12-lane collector–express system from the Credit River to just east of Winston Churchill Boulevard, while another 12-lane collector–express system was built from just west of the Highway 407 interchange to just east of James Snow Parkway, due to the overpasses for the 407 and the adjacent Winston Churchill Boulevard not being constructed wide enough to accommodate a continuous collector–express system underneath.
Sections of the new expansion were opened overnight throughout the second half of 2022. On August 13 and 14, 2022, the westbound express lanes opened between Highway 407 and James Snow Parkway. The remainder of the westbound lanes, between the Credit River and Winston Churchill Boulevard, were opened several months later on November 13 and 14. The eastbound express lanes between James Snow Parkway and Winston Churchill Boulevard were opened a week later on November 18 and 19, and from Winston Churchill Boulevard to the Credit River on November 29 and 30. The HOV lanes in both directions were opened on December 9, 2022. The remaining works of this project, including final layers of paving, culvert works, and carpool lots were completed on October 19, 2023, thus completing a 14-year project that expands the highway from Highway 410 to Regional Road 25 that started on August 19, 2009.


### COMPASS cameras
In early 1991, Highway 401 was equipped with a traffic camera system called COMPASS.
Using closed-circuit television cameras, vehicle detection loops and LED changeable-message signs, COMPASS enables the MTO Traffic Operations Centre to obtain a real-time assessment of traffic conditions and alert drivers of collisions, congestion and construction.
The system stretches from the Highway 403 / 410 interchange in Mississauga to Harwood Avenue in Ajax.


### Advantage I-75
Between June 1990 and 1998, Highway 401 and Interstate 75 were used for a pilot project named Advantage I-75 to test the reliability and versatility of an automated tracking system for transport trucks. Termed "MACS" (Mainline Automated Clearance System), it allows a truck to travel from Florida to Ontario without a second inspection.
MACS was initially tested at two truck inspection stations in Kentucky, with transponders installed in 220 trucks. Exact time, date, location, weight and axle data were logged as a truck approached an equipped station.
Following initial tests, MACS was deployed at every inspection station along I-75 from Miami to Detroit, and along Highway 401 from Windsor to Belleville in 1994. The project demonstrated the effectiveness of electronic systems in enforcing freight restrictions without delaying vehicles, while alleviating security fears such systems could be compromised. The concept has since been applied to many parts of Canada, including Highway 407's electronic tolling system.


### "Carnage Alley"
The section of Highway 401 between Windsor and London has often been referred to as Carnage Alley, in reference to the numerous crashes that have occurred throughout its history. The term became more commonplace following several deadly pileups during the 1990s. The narrow and open grass median was an ineffective obstacle in preventing cross-median collisions. The soft shoulders consisted of gravel, with sharp slopes which were blamed for facilitating vehicle rollovers.
The nature of that section of highway, described as a mainly straight road with a featureless agricultural landscape, was said to make drivers feel less involved and lose focus on the road. In winter, the area between Woodstock and Chatham is also subject to sudden snow squalls from lake-effect snow.
Several collisions have resulted from motorists deviating from their lane and losing control of their vehicles.
Various other names, including The Killer Highway circulated for a time,
but Carnage Alley became predominant following an 87-vehicle pile-up on September 3, 1999 (the start of Labour Day weekend), the worst in Canadian history, that resulted in eight deaths and 45 injured individuals.

Only a few days prior, then-Transportation Minister David Turnbull had deemed the highway "pleasant" to drive.
On the morning of September 3, the local weather station reported clear conditions due to a malfunction, while a thick layer of fog rolled onto the highway. Dozens of vehicles, including several semi-trailers, quickly crashed into each other shortly after 8 a.m., one following another in the dense fog, with collisions in both directions at that segment of Highway 401, although no vehicles crossed the highway's median.
Immediately following the crash, the MTO installed paved shoulders with rumble strips
and funded additional police to patrol the highway, a move criticized as being insufficient.
Beginning in 2004, 46 km (29 mi) of the highway was widened from four asphalt lanes to six concrete lanes, paved shoulders were added, and a concrete Ontario Tall Wall median was installed,
which was the solution the Canadian Automobile Association promoted in 1999. Interchanges were improved and signage was upgraded as part of a five-phase project to improve Highway 401 from Highway 3 in Windsor to Essex County Road 42 (formerly Highway 2) on the western edge of Tilbury.
From 2008 to 2010, with joint funding from the provincial and federal governments, the section of Highway 401 from Dougall Parkway (former Highway 3B) to Provincial Road (former Highway 98) was widened to six lanes, necessitating the replacement of several underpasses including those of Walker Road and Provincial Road. The loop ramp from Provincial Road southbound to Highway 401 eastbound was removed, although the rest of the interchange remained unchanged. As part of that project, the Dougall Parkway split with Highway 401 was reconfigured, replacing a one-lane 1950s-era underpass tunnel with a modern high-speed flyover ramp from Dougall Parkway that joins the eastbound freeway lanes on the right-hand side. The old interchange had reduced Highway 401 eastbound traffic to one lane as it merged with the left-hand on-ramp from Dougall Parkway, whereas the new interchange allows three lanes per direction of Highway 401 to pass through. West of the reconstructed Dougall Parkway onramp, Highway 401's grass median was replaced with a concrete barrier but continued to be striped for two lanes per direction until the North Talbot Road overpass was replaced and Phase One of the Highway 401 extension (Rt. Hon. Herb Gray Parkway, formerly Windsor-Essex Parkway) (see below) opened in 2015, allowing for a continuous six lane freeway cross-section.


### Highway of Heroes
On August 24, 2007, the MTO announced the stretch of Highway 401 between Glen Miller Road in Trenton and the intersection of the Don Valley Parkway and Highway 404 in Toronto would bear the additional name Highway of Heroes (French: Autoroute des héros), in honour of Canadian soldiers who have died,
though Highway 401 in its entirety remains designated as the Macdonald–Cartier Freeway.
This length of the highway is often travelled by a convoy of vehicles carrying a fallen soldier's body, with his or her family, from the military airport at CFB Trenton to the coroner's office at the Centre of Forensic Sciences in Toronto. Since 2002, when the first fallen Canadian soldiers were repatriated from Afghanistan, crowds have lined the overpasses to pay their respects as convoys pass.

The origin of the name can be traced to an article in the Toronto Sun on June 23, 2007, by columnist Joe Warmington, in which he interviewed Northumberland photographer Pete Fisher. Cobourg resident Ron Flindall was responsible for organizing the first bridge salutes following the loss of four soldiers on April 18, 2002.
Warmington described the gathering of crowds on overpasses to welcome fallen soldiers as a "highway of heroes phenomena".
This led a Cramahe Township volunteer firefighter to contact Fisher on July 10 about starting a petition, leading Fisher to publish an article which was posted to the Northumberland Today website.
The online article eventually caught the attention of London resident Jay Forbes. Forbes began a petition, which received over 20,000 signatures before being brought to the Minister of Transportation on August 22.
Following the announcement on August 24, the provincial government and MTO set out to design new signs. The signs were erected and unveiled on September 7, and include a smaller reassurance marker (shield), as well as a larger billboard version.
On September 27, 2013, the Highway of Heroes designation was extended west to Keele Street in Toronto, to coincide with the move of the coroner's office to the new Forensic Services and Coroner's Complex located at Highway 401 and Keele Street.


### Highway improvements and safety concerns
#### London and Kitchener
Between 2006 and 2008, Highway 401 was widened from four to six lanes between Highway 402 and Wellington Road in London. This included reconfiguring the Wellington Road interchange from a cloverleaf to a Parclo A4 while replacing the original 1956 overpass with a longer and wider structure.
In November 2010, the widening of Highway 401 from four to six lanes between Woodstock and Kitchener was completed after many years of planning and construction.
The project included the installation of a tall-wall median barrier, straightening curves and adding additional interchanges on the freeway, allowing it to be easily vacated in an emergency event.


#### Greater Toronto Area
Beginning in 1998, several projects were initiated on Highway 401 within Toronto. The freeway's pavement through the city was resurfaced. The most significant construction work was widening the route from six to eight lanes though the Highway 427 interchange in 2005, which necessitated the replacement of the Highway 27 underpass, although the rest of the junction's flyovers could accommodate the expansion. Some projects have been completed during overnight construction projects, including the widening and rehabilitation of the Hogg's Hollow bridge,
the replacement of the original gantries throughout the collector-express system, and rehabilitating the flyover ramps of the Highway 401 / 400 interchange.

On August 10, 2008, following a series of explosions at a propane facility in Toronto, Highway 401 was closed between Highway 400 and Highway 404 as a precautionary measure, the largest closure of the highway in its history.
The highway remained closed until 8 p.m., though several exits near the blast remained closed thereafter.
In Oshawa, exit 416 (Park Road) was replaced by a new interchange at exit 415 (Stevenson Road). The contract, which began September 7, 2005, included the interchange and the resurfacing of 23.4 km (14.5 mi) of the highway between Oshawa and Highway 35 / Highway 115.
The westbound ramps were opened in mid-September 2007
and the eastbound ramps in mid-2009. The resurfacing was completed mid-2010.

In 2013, as a prerequisite to construction of the West Durham Link which was eventually numbered as Highway 412, the section of Highway 401 near the Lake Ridge Road overpass was shifted northward on a new alignment away from the parallel railway lines to allow sufficient right-of-way for the interchange to the new route. The existing Lake Ridge Road overpass was also replaced by a new longer structure that would span both the existing and new alignments of Highway 401 as well as the railways. Once the new alignment of Highway 401 was opened for both directions, the old Highway 401 right-of-way was used for the semi-directional onramp from Highway 401 eastbound to Highway 412 as well as an offramp to Lake Ridge Road. The interchange to Highway 412 opened on June 20, 2016.
Highway 401's interchange with Highway 418 (East Durham Link) opened on December 9, 2019, alongside the opening of an eastern extension of Highway 407.


### Rt. Hon. Herb Gray Parkway
Highway 401 as originally completed did not have direct access to the Canada–United States border, as upon entering the Windsor city limits the route terminated at a semi-directional Y interchange where the freeway defaulted into Highway 3. Traffic continuing to the border had to follow the non-express Highway 3 routing along Talbot Road and Huron Church Road which had a dozen traffic lights. The approach to the Ambassador Bridge was heavily built up making it impractical to twin the existing crossing and reconstruct the approach as a freeway to meet growing demand for cross-border traffic, even though Ambassador Bridge owner Matty Moroun had long lobbied for this.
In 2004, a joint announcement by the federal government of the United States and Government of Canada confirmed a new border crossing would be constructed between Detroit and Windsor. The Detroit River International Crossing (DRIC) was formed as a bi-national committee to manage the project.
The MTO took advantage of this opportunity to extend Highway 401 to the Canada–US border and began an environmental impact assessment on the entire project in late 2005.
The City of Windsor also hired New York traffic consultant Sam Schwartz to design a parkway to the border. Schwartz's proposal would eventually inspire the DRIC's own design, but his route was not chosen, with the DRIC opting instead to take a northern route.
On February 8, 2008, the MTO announced it had begun purchasing property south of the E. C. Row Expressway, upsetting many area residents who had purchased properties in the years prior.
On March 3, 2008, the Michigan Department of Transportation and the MTO (in partnership with Transport Canada, the Federal Highway Administration of the United States and the Detroit River International Crossing group) completed a joint assessment on the soils along the Detroit River and determined they could indeed support the weight of a new bridge; the stability of the underlying soil and clay and the impact of the nearby Windsor Salt Mine had caused concern for all parties involved in the project.
Despite protest from area residents,
as well as a dismissed lawsuit from Ambassador Bridge owner Matty Moroun, it was announced on May 1, 2008, that a preferred route had been selected for the Highway 401 extension which would be named the Windsor–Essex Parkway. On November 28, 2012, the Ministry of Transportation announced a Federal Order in Council was passed to change the name of the parkway to the "Rt. Hon. Herb Gray Parkway", in honour of the Right Honourable Herb Gray, a Member of Parliament from Windsor.
The Highway 401 extension (Rt. Hon. Herb Gray Parkway, formerly Windsor-Essex Parkway, MTO internal designation of Highway 7901) would first run parallel to the realigned Highway 3 (Talbot Road and Huron Church Road) from a new interchange at the former end of Highway 401 to the E. C. Row Expressway. The Highway 401 extension would then turn westward and continue between the opposing carriageways of the E. C. Row Expressway for 2 km (1.2 mi) (requiring that the E. C. Row's eastbound carriageway between Matchette Road and Huron Church Road be shifted so the median can accommodate the Highway 401 extension), with slip ramps linking the E. C. Row to the bridge-bound Highway 401. At the Ojibway Parkway, the Highway 401 extension would turn northwest and follows a new alignment to the border. The Highway 401 extension would have six through-lanes, and the section parallel to Highway 3 is below-grade with 11 covered tunnels ranging from 120 to 240 metres (390 to 790 ft) in length. The Highway 401 extension features 120 hectares (300 acres) of green space and over 20 kilometres (12 mi) of recreational trails, with seven bridges and two tunnels separating the trails from roads. Interpretive signage includes information about First Nations in Canada, Tallgrass prairie and the Carolinian landscape.
Initial construction of a noise barrier from North Talbot Road to Howard Avenue began in March 2010; full construction began on August 19, 2011. The original North Talbot Road overpass was demolished on August 20, 2011, to make way for the widening of the existing freeway from four through-lanes, however the replacement bridge was closed in December 2013 due to faulty concrete girders and rebuilt by May 2014. Progress on the freeway extension was delayed as the construction consortium disposed of over 500 concrete girders (including the 320 girders already installed), after a Ministry of Transport investigation found that these girders manufactured from a partnership of Freyssinet and Tierra Armada did not meet standard, and replaced by those from Prestressed Systems Inc. at no cost to the public.
The Highway 401 extension was built on a new alignment while Highway 3 (Talbot Road) was also rerouted, so the old interchange of Highway 401 at Highway 3 remained in service until it was bypassed once the extension and replacement interchange opened. The old interchange as well as the signalized intersection at Outer Drive has since been removed. In early 2015, it was announced the first phase of the Highway 401 extension would open to traffic between Highway 3 and Labelle Street (near the E. C. Row Expressway) in the spring;
an 8-kilometre (5 mi) section was opened to traffic on June 28, 2015, extending Highway 401 as far west as the E. C. Row Expressway, the first new segment of Highway 401 to be opened since the Thousand Islands Parkway bypass in 1968. The first phase of the Highway 401 extension provides an express bypass to much of (but not all of) Highway 3 en route to the Ambassador Bridge. The E. C. Row still lacked direct access to the London-bound portion of Highway 401, however, the indirect connection using Huron Church Road was reduced to a short distance (with one traffic light) between the Parclo A4 interchange with the E. C. Row and the slip ramps to the first phase of the Highway 401 extension. The second phase of the Highway 401 extension to Ojibway Parkway was opened on November 21, 2015
completing the Highway 401 extension as far as the planned bridge approach and border plaza. Crossing the Detroit River, the new bridge will extend Highway 401 across the US border to a connection through Delray to I-75.
Construction on projects related to the Gordie Howe International Bridge began in 2015 with an initial completion date in 2019–20.
The "Bridging North America" consortium was selected to build the bridge in July 2018, with construction beginning immediately.
The Gordie Howe International Bridge is expected to be completed in early 2026.


### Major projects 2006–2024
#### Southwestern Ontario
In Southwestern Ontario, several improvements are under way to provide six lanes on Highway 401 from Windsor to Toronto,
in response to the higher-than-average accidents occurring on the "Carnage Alley" stretch including the 1999 Labour Day weekend pileup.
West of  Essex County Road 42 on the west of Tilbury, the highway has been widened to six lanes with a concrete divider in anticipation of the Rt. Hon. Herb Gray Parkway. As of 2022 the MTO has initiated studies on widening 118 km of the freeway from four to six lanes between Tilbury and London through four contracts.

Around London the traffic volumes are expected to increase considerably, so the province established an extensive plan to widen and reconstruct the London corridor between 2006 and 2021. This included building a new interchange with Wonderland Road which opened in November 2015 to help improve access to Highway 401 westbound from the city's southwest end and involved replacing the Westminster Drive overpass to allow the highway to be widened. A reconstruction of the outdated cloverleaf interchange at Colonel Talbot Road and widening Highway 401 from four to six lanes between Highway 4 and Highway 402 is also proposed. The MTO is also planning on widening Highway 401 from six to eight lanes through part of the London corridor. Alongside the extension of the Veterans Memorial Parkway (formerly Airport Road) south of the freeway to Wilton Grove Road in 2017, the Parkway's trumpet interchange was reconfigured from a trumpet to a Parclo with the replacement overpass able to accommodate future freeway expansion.
In the Kitchener/Cambridge area, the widening of Highway 401 from six to twelve lanes from Highway 8 (King Street) to Highway/Regional Road 24 (Hespeler Road) commenced on June 8, 2015, and concluded in summer 2019.  The second phase of expansion scheduled for 2019-21 saw Highway 401 widened from six to ten lanes between Highway/Regional Road 24 and Townline Road. At the interchange with Highway 24 (Hespeler Road), the span carrying northbound traffic which opened in 1989 was torn down on May 1, 2021, with both directions of Highway 24 traffic temporarily relocated to the overpass originally built in 1960 and designated for southbound drivers since 1989, as a new overpass was constructed to accommodate Highway 401's expanded cross-section. The new HOV lanes from Highway 8 to Townline Road opened on December 22, 2023. Just west of the Highway 8 interchange, the freeway's existing six-span Grand River Bridges (opened in 1960, restriped in the early 1990s from two to three lanes per direction) are being replaced with two new four-span structures, a project scheduled for completion in 2025. The new Grand River bridges are a prerequisite to the planned expansion of the Y-junction between Highway 401 and Highway 8 Expressway from a partial to an all-directional interchange, with proposed flyover ramps linking eastbound Highway 401 to Waterloo-bound Highway 8 and the opposite movement to bypass the existing non-express connection using King Street.


#### Greater Toronto Area
Expansion in Durham includes widening the highway to 12 lanes, and extending the collector-express system from its end at Brock Road in Pickering to Lake Ridge Road in Whitby.
A Transportation Environmental Study Report was completed on widening highway 401, extending the collector-express system easterly through to the Highway 412 interchange in Whitby, then ten lanes easterly to Liberty Street in the Municipality of Clarington. The assessment was completed in March 2015.
To support this widening, all of the original overpasses dating back in the 1940s and 1950s built through Whitby and Oshawa were replaced with new overpasses as part of contemporary highway safety standards and to allow for a future highway widening.


#### Eastern Ontario
East of Durham, the MTO widened parts of Highway 401 to six lanes. Two bridges have been widened in advance of an eventual widening to six lanes of the highway including the bridges over the Trent River in Trenton, as well as the Salmon River bridge between Belleville and Napanee.
By 2020, the highway was widened to six lanes for 9 km (5.6 mi) through Kingston between exits 611 and 623, 16 years after it started back in 2004. Construction began in 2014 to expand the highway to six lanes approximately five kilometres (3.1 mi) east of exit 474 in Cobourg.


### Proposed tunnel
On September 25, 2024, Premier Doug Ford announced that the Ontario government has launched the technical evaluation for the proposed construction of a new driver and transit tunnel expressway under Highway 401.


## Services
Highway 401 features 19 ONroute service centres operated under contract from the Ministry of Transportation. They provide a place to park, rest, eat and refuel 24 hours a day.
Service centres along Highway 401 were first announced in 1961 following public outcry over the lack of rest stops. The centres were originally leased to and operated by several major gasoline distributors; however, those companies chose not to renew their leases as the terms ended. The centres were also of an outdated design that could not keep up with growing traffic, despite limited renovations in the early 1990s such as at the Woodstock and Cambridge North/South sites. In response, the MTO put the operation of the full network of service centres out for tender, resulting in a 50-year lease agreement in 2010 with Host Kilmer Service Centres, a joint venture between hospitality company HMSHost (a subsidiary of Autogrill) and investment company Kilmer van Nostrand.
Seventeen of the centres along Highway 401 have been entirely redeveloped. Prior to the deal with Host Kilmer Service Centres, the centres at Newcastle and Ingersoll were rebuilt in the late 1990s and did not require further reconstruction since their design is modern enough. In Mississauga (just east of Winston Churchill Boulevard), a centre with a unique layout opened in 1991 but closed in 2006, being demolished in 2010 to accommodate the freeway's widening to a collector-express system as well as making way for an OPP station. Work on rebuilding 15 of the 17 service centres began in late 2009 or early 2010. The new service centres, opened in phases beginning in July 2010, feature a Canadian Tire gas station, an HMSHost-operated convenience store known as "The Market", as well as fast food brands such as Tim Hortons, A&W, Pizza Pizza, Extreme Pita, KFC, Taco Bell, Big Smoke Burger and Burger King.


## Exit list
The following table lists the major junctions along Highway 401, as noted by the Ministry of Transportation of Ontario. 


## See also
Heavy Rescue: 401—a documentary/reality series on Discovery Channel in Canada and The Weather Channel in the United States highlighting the operations of heavy rescue and recovery towing businesses along the highway and nearby roads.
Southern Ontario Transportation


## Explanatory notes
## Citations
## General bibliography
### Books
### Maps
## External links
### Photos and additional information
Highway 401 at thekingshighway.ca
Highway 401 at OntHighways
Highway 401 at vintagekingshighways.com (archived December 30, 2009)