Highway 401 is the primary and busiest thoroughfare across Southern, Central and Eastern Ontario.The highway was completed the late 1960s to bypass Toronto, but with the post-War growth of automobile plants, other manufacturing, and the construction of vast bedroom communities around Toronto (and the expansion of its airport), the 401 is an important economic corridor. used by millions of motorists. The 401 section between Weston Road and Highway 400 carries over 420,000 vehicles on an average day, making it North America’s busiest highway.
By the 1930s a two lane Highway 2 stretched across the province near the shore of Lake Ontario. By World War II and new four lane Highway 2A was built further away from the lake initially between Westhill (Scarborough) and Oshawa. In 1952, the 400-series of highway standards was introduced.
The most important link in Highway 401 was the Toronto Bypass. This critical section of the new highway ran just to the north of the urbanized area of Toronto. The Toronto Bypass was completed in 1956 after several years of construction. It ran from Highway 27 (Etobicoke) to West Hill (in Scarborough).
By the late 1950s, congestion was becoming a serious problem on Highway 401 across Toronto. The Toronto Bypass was widened to a minimum of twelve lanes (six lanes per direction) from Islington Avenue to Neilson Road during the 1960s and early 1970s, with a median between collector and express lanes to separate local traffic from through traffic. Motorists could only enter or exit the highway from the local/collector lanes (except at a handful of freeway interchanges) but motorists can switch between the express and collector lanes at strategic intervals.
The final section of Highway 401 was completed between Gananoque and Brockville in 1968, completing this 818 km controlled-access freeway across the southern half of Ontario. In the areas east and west of Toronto, service stations and restaurants were built just off the highway, so travellers would never need to exit the highway, further speeding the flow. The highway was officially rededicated as the Macdonald-Cartier Freeway in 1965, to commemorate two of Canada’s Fathers of Confederation, Sir John A. Macdonald and Sir George Etienne Cartier. Special commemorative highway signs were placed at every entrance to the highway, but over an era of government cost-cutting, these signs were phased out in the 1990s.