The 400-series highways are a network of controlled-access highways in 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 Autoroute system of neighbouring Quebec, and are regulated by the Ministry of Transportation of Ontario (MTO). The 400-series designations were introduced in 1952, although Ontario had been constructing divided highways for two decades prior. Initially, only Highways 400, 401 and 402 were numbered; other designations followed in the subsequent decades. To this day, not all controlled-access highways in Ontario are a part of the 400-series highway network. The network is situated almost entirely in Southern Ontario, although Highway 400 extends into the more remote northern portion of the province.
Modern 400-series highways have high design standards, speed limits of 100 kilometres per hour (62 mph), with a 110 kilometres per hour (68 mph) limit on select stretches, and various collision avoidance and traffic management systems. The design of 400-series highways has set the precedent for a number of innovations used throughout North America, including the parclo interchange and a modified Jersey barrier design known as the Ontario Tall Wall. As a result, they currently experience one of the lowest accident and fatality rates comparative to traffic volume in North America.
