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 (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 kilometres (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 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.
