File:Ontario Highway 401 Between Kingston and Brockville, Ontario (30069391395).jpg

King's Highway 401, commonly referred to as Highway 401 and also known by its official name as the Macdonald–Cartier Freeway or colloquially as the four-oh-one, is a 400-series highway in the Canadian province of Ontario. It stretches 828.0 kilometres (514.5 mi) from Windsor in the west to the Ontario–Quebec border in the east. The part of Highway 401 that passes through Toronto is the busiest highway in the world, and one of the widest. Together with Quebec Autoroute 20, it forms the road transportation backbone of the Quebec City–Windsor Corridor, along which over half of Canada's population resides and is also a Core Route in the National Highway System of Canada. The entire route is maintained by the Ministry of Transportation of Ontario (MTO) and patrolled by the Ontario Provincial Police. The posted speed limit is 100 km/h (62 mph) throughout its length, unless posted otherwise. 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 1965 it was given a second designation, the Macdonald–Cartier Freeway, in honour of the 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-km length. On August 24, 2007, the portion of the highway between Glen Miller Road in Trenton and the Don Valley Parkway / Highway 404 Junction in Toronto was designated the Highway of Heroes, as the road is travelled by funeral convoys for fallen Canadian Forces personnel from CFB Trenton to the coroner's office in Toronto. 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 at the Humber River Hospital.

In 2011, construction began on a westward extension called the Right Honourable Herb Gray Parkway. This new route follows, but does not replace, former Highway 3 between the former end of the freeway and the E. C. Row Expressway, at which point it turns and parallels that route towards the site of the future Gordie Howe International Bridge. An 8-kilometre (5 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. Elsewhere in Ontario, plans are under way 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 will also be extended west through Mississauga to Milton and east through Ajax and Whitby.

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 2008 analysis stated that the annual average daily traffic (AADT) count between Weston Road and Highway 400 in Toronto was approximately 450,000, while a second study estimates that over 500,000 vehicles travel that section on some days. This makes it the busiest roadway in North America, 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 busiest truck route in the world, carrying 60 percent of vehicular trade between Canada and the US.

Highway 401 also features the busiest multi-structure bridge in North America, located at Hogg's Hollow in Toronto. The four bridges, two for each direction with the collector and express lanes, carried an average of 373,700 vehicles daily in 2006. 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 Autoroute 20 at the Ontario–Quebec border.

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