King's Highway 427 (pronounced "four twenty-seven"), also known as Highway 427 and colloquially as the 427, is a 400-series highway in the Canadian province of Ontario that runs from the Queen Elizabeth Way (QEW) and Gardiner Expressway in Toronto to Major Mackenzie Drive (York Regional Road25) in Vaughan. It is Ontario's second busiest freeway by volume and the third busiest in North America, behind Highway401 and Interstate 405 in California. Like Highway401, a portion of the route is divided into a collector-express system with twelve to fourteen continuous lanes. Notable about Highway427 are its several multi-level interchanges; the junctions with the QEW/Gardiner Expressway and Highway401 are two of the largest interchanges in Ontario and were constructed between 1967 and 1971, while the interchanges with Highway 409 and Highway 407 were completed in 1992 and 1995, respectively. Highway427 is the main feeder to Toronto Pearson International Airport from the north and south. However, while much of the traffic from Highway407, Highway401 (eastbound), and the QEW/Gardiner Expressway makes use of the freeway for airport access, it also serves as a major traffic route for the western portion of Toronto (Etobicoke), the northeastern portion of Mississauga (Malton), the southeastern portion of Brampton (Claireville), and the western portion of Vaughan (Woodbridge). The section between Highway 401 and Dundas Street is a heavily traversed transit corridor; the stretch between Burnhamthorpe and Rathburn saw an average of over 400,000 vehicles and over 5,000 buses per day in 2016. GO Transit, MiWay, and the Toronto Transit Commission (TTC) all operate express buses along this section of the highway. First designated in 1972, Highway427 assumed the recently completed 12-lane collector-express freeway of Highway 27, as well as a short freeway north of Highway401 known as the Airport Expressway. Both routes were upgraded throughout the 1950s and 1960s, eventually becoming intertwined into the present configuration in 1972. The freeway was extended north from Pearson Airport to Highway7 over the following twenty years. Construction of an extension north to York Regional Road 25 (Major Mackenzie Drive) began in 2017 and was opened on September18, 2021, following a legal dispute between the construction consortium and the provincial government. Route description Highway 427 is the second-busiest freeway in Canada with an average of 300,000 vehicles that use it between the QEW and Highway401 per day. The section between Burnhamthorpe Road and Rathburn Road has an annual average daily traffic (AADT) count of