King's Highway 401, also known by its official name as the Macdonald–Cartier Freeway and colloquially as the four-oh-one, is a 400-series highway in the Canadian province of Ontario. It stretches 817.9 kilometres (508.2 mi) from Windsor to the Quebec border. 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. The entire route is maintained by the Ministry of Transportation of Ontario (MTO) and patrolled by the Ontario Provincial Police.