400-series Highways

The 400-series highways are a network of controlled-access highways throughout the southern portion of the Canadian province of Ontario, forming a special subset of the provincial highway system. They are analogous to the Interstate Highway System in the United States or the British Motorway network in the United Kingdom. Modern 400-series highways have high design standards, speed limits of 100 kilometres per hour (62 mph), and various collision avoidance and traffic management systems. 400-series highway design has set the precedent for a number of innovations used throughout North America, including the parclo interchange and the modified Jersey barrier design known as an Ontario Tall Wall.

The province's baseline standard for the construction of a freeway is an average traffic count of 10,000 vehicles per day. However, other factors are considered as well. To promote economic development in a disadvantaged region (e.g., the current extension of Highway 400 to Northern Ontario), a 400-series highway may be built where the existing highway's traffic counts fall below 10,000. As well, for environmental, budgetary or community reasons, some proposed 400-series highways (e.g., the Highway 400 extension in Toronto from Highway 401 to the Gardiner Expressway that was cancelled in the 1970s) have not been built, even where an existing highway's traffic counts exceed the standard.