Ontario Highway 401

King's Highway 401, commonly referred to as Highway 401 and officially known 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 (514 mi) freeway, travelling from the Gordie Howe International Bridge at 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 Quebec's 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.

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 the highway were 11.8, 54.7, and 41.2 kilometres (7.3, 34.0, and 25.6 mi), respectively. In November 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 Highway 401 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, 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 Gordie Howe International Bridge, which has opened as of July 27, 2026. An eight-kilometre (5.0 mi) section of the parkway, east of the E. C. Row interchange, opened to traffic on June 28, 2015, with the remaining section completed and opened on November 21. The widening of Highway 401 from Highway/Regional Road 8 in Kitchener east to Townline Road in Cambridge to at least ten lanes was completed on 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 Ajax, Whitby, and 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.

The speed limit is 110 km/h (68 mph), except between Highway 427 and Highway 404 in Toronto, where the speed limit is 100 km/h (62 mph), and west of Concession Road 9 in Windsor, where the speed limit is 80 km/h (50 mph). The route is patrolled by the Ontario Provincial Police (OPP) and maintained by the Ministry of Transportation of Ontario (MTO).