The Ontario Provincial Highway Network consists of all the roads in Ontario maintained by the Ministry of Transportation (MTO), including those designated as part of the King's Highway, secondary highways, and tertiary roads. Components of the system—comprising of roads and 2,880 bridges—range in scale from Highway 401, the busiest highway in North America, to unpaved forestry and mining access roads. The longest highway, Highway 17, is nearly long, while the shortest, Highway 2, is less than a kilometre. Some roads are unsigned highways, lacking signage to indicate their maintenance by the MTO; these may be remnants of highways that are still under provincial control whose designations were decommissioned, roadway segments left over from realignment projects, or proposed highway corridors.

Predecessors to today's modern highways include the foot trails and portages used by Indigenous peoples in the time before European settlement. Shortly after the creation of the Province of Upper Canada in 1791, the new government under John Graves Simcoe built overland military roads to supplement water-based transportation, including Yonge Street and Dundas Street. At the time, road construction was under the control of the township and county governments. Local township roads were financed and constructed through a statute labour system that required landowners to make improvements in lieu of taxes. Private companies constructed corduroy and later plank roads and charged tolls in the second half of the 19th century. The rising popularity of the bicycle led to the formation of the Ontario Good Roads Association, which advocated for the improvement of roads and recreation as the automobile rose to prominence.

By the early 20th century, the province had taken interest in road improvement and began funding it through counties. The increasing adoption of the automobile resulted in the formation of the Department of Public Highways of Ontario (DPHO) in 1916. The passing of the Canada Highways Act in 1919 resulted in the establishment of a provincial network of highways. The DPHO assigned internal highway numbers to roads in the system, and in 1925, the numbers were signposted along the roads and marked on maps. In 1930, provincial highways were renamed King's Highways and the familiar crown route markers created. The DPHO was also renamed the Department of Highways (DHO).

The 1930s saw several major depression relief projects built by manual labour, including the first inter-city divided highway in North America along the Middle Road, which would become the Queen Elizabeth Way in 1939. In 1937, the DHO merged with the Department of Northern Development, extending the highway network into the Canadian Shield and Northern Ontario. Significant traffic engineering and surveying through the war years, during which construction came to a near standstill, led to the planning and initial construction of controlled-access highways. The 400-series highways were built beginning in the late 1940s and numbered in 1952.

The vast majority of modern road infrastructure in Ontario was built throughout the 1950s, 1960s, and early 1970s. The cancellation of the Spadina Expressway and the introduction of the Environmental Assessment Act in the 1970s resulted in a decline in new highway construction in the decades since. In the late 1990s, nearly of provincial highways were transferred, or "downloaded" back to lower levels of government. Few new provincial highways have been built in the early years of the 21st century, although several major infrastructure projects including the Herb Gray Parkway and expansion of Highway 69 have proceeded. Recent construction has included the controversial Bradford Bypass and Highway 413.

Naming, signage and regulations

In Ontario, all public roads are legally considered highways under the Highway Traffic Act (HTA), which sets forth regulations for traffic, or the rules of the road. The Public Transportation and Highway Improvement Act (PTHIA) sets forth the regulations concerning roads that are under the authority of the Province of Ontario. The act distinguishes and sets out the applicability of the HTA to provincial highways, which are designated as part of The King's Highway (primary), a secondary highway, or a tertiary road. Tertiary roads may also be designated as a resource road, allowing for vehicles otherwise prohibited from public roads. Industrial roads are privately owned routes with which the MTO has entered an agreement to improve or allow public access, and are not considered part of the provincial highway network. The 407 ETR is likewise not considered part of the provincial highway network. While it is still subject to the rules set forth by the HTA, it is otherwise governed independently under the legislation of the Highway 407 Act.

Speed limits on provincial highways are legislated by sections of the road, and vary between and . Freeways, including the 400-series highways, are generally signed at , although sections exist that are signed lower.

Marker design

Ontario uses two distinct shapes of signage to mark the King's Highways. Confirmation markers, or reassurance markers, are utilized along the designated road to confirm (near intersections) or reassure (elsewhere) drivers that they are on the correct route. The markers, known as shields, feature the route number within an outline in the shape of a shield, topped by a St Edward's Crown. In other cases, particularly when approaching the junction of another highway, a square crown marker is used, featuring the route number within an outline of the St Edward's Crown, paired with an arrow plate; exit signs on freeways and at major junctions also use this crown symbol. For secondary highways, the route number is within an outline of an isosceles trapezoid, while tertiary roads place the number within an outline of a rectangle. When these markers appear along or at an intersection with the indicated highway, they feature black text on a reflective white background. There are two exceptions to this: The QEW, which features blue text on a yellow background; and the provincially maintained section of the tolled Highway 407, which feature white text on a blue background, with an orange plate with TOLL below in black. Signs prior to 1993 had the words "The King's Highway" below the crown, but current versions do not have the words.

In addition to regular highway markers, there are trailblazers, which indicate a route towards that highway. These are the same shape as their corresponding highway marker. Trailblazers for the King's Highway, which can be shields or crowns, feature white text on a reflective green background, with the exception of trailblazers for the QEW, which feature yellow text on a reflective blue background. For secondary highways, trailblazers simply add the word "TO" above the route number.

Since August 2004, "Highway of Heroes" shields featuring a diagram of a poppy have been posted along Highway401 between Toronto and CFB Trenton. These were erected to honour fallen Canadian soldiers, whose bodies were repatriated from Afghanistan in funeral convoys along that stretch of the highway.

Classification Ontario has several distinct classes of highways:

The King's Highway

The King's Highway is the primary highway network of Ontario, and constitutes the majority of the principal inter-urban roadways in the province. As a whole, it is referred to in the singular form as opposed to as a group of its parts (i.e. "the King's Highway", not "the King's Highways"). Individual highways are known as "part of the King's Highway" or "the King's Highway known as n". However, in common parlance they are simply referred to as "Highway n". Ontario highways rank second safest in North America for fatality rates, with 0.55 fatalities per 10000 licensed drivers in 2019. The phrase "King's Highway" is used regardless of the gender of the monarch.

The 400-series highways and the QEW form the backbone of the King's Highway, with other routes numbered from 2 to 148. The Ministry of Transportation never designated a Highway 1. Some highway numbers are suffixed with a letter A ("alternate route"), B ("business route"), or N ("new route"). In the past, there have also been routes with C and S ("scenic route") suffixes. The entire King's Highway network is fully paved. The term "the King's Highway" was first adopted in place of "provincial highway" in 1930, and signs similar to the current design replaced the previous triangular signs at that time. Some legislative acts refer to roads that are under the jurisdiction of the province as "provincial highways".

400-series

The 400-series highways are a network of controlled-access highways throughout the southern portion of Ontario, forming a special subset of the provincial highway network. They are analogous to the Interstate Highway System in the United States or the Autoroute system of neighbouring Quebec, and are regulated by the MTO. The 400-series designations were introduced in 1952, although Ontario had been constructing divided highways for two decades prior. Initially, only Highways400, 401 and 402 were numbered; other designations followed in the subsequent decades.

While older freeways have some lapses in safety features, contemporary 400-series highways have design speeds of , speed limits of , with lower limits on a few sections, various collision avoidance and traffic management systems, and several design standards adopted throughout North America. Of note are the Ontario Tall Wall median barrier and the Parclo A-4 interchange design, the latter which became standard in the design for the widening of Highway 401 through Toronto in 1962. The Institute of Traffic Engineers subsequently recommended this design to replace the cloverleaf interchange throughout North America.

Secondary

Secondary highways exist solely within the districts of Northern Ontario that lack a county road system, to which they are analogous. For the purposes of MTO clasification, "Northern Ontario" refers to Districts of Parry Sound and Nipissing and those farther north - those Districts obtaining highway funding from Ontario Ministry of Northern Development and Mines. They generally serve to connect remote communities to the King's Highway, or to interconnect the King's Highway. A few secondary highways remain gravel-surfaced, although most have been paved. The speed limit on nearly all of these routes is , although Highway 655 is posted at .

The Secondary Highway system was introduced in 1956 to service regions in Northern and Central Ontario, though it once included a route as far south as Lake Ontario. Many routes that would become secondary highways were already maintained by the province as development roads prior to being designated. Since 1998, none have existed south of the districts of Parry Sound and Nipissing. Secondary highways are numbered in the 500s and 600s, with existing highways numbered between 502 and 673.

Tertiary Tertiary roads are remote routes entirely within Northern Ontario that provide access to resources (e.g. mining and forestry). Tertiary roads are numbered in the 800s, with the five existing highways numbered between 802 and 811. Most of these roads are gravel-surfaced and of low-standard. The speed limit on these routes is , although design standards generally prevent such. Unlike other roads in the Provincial Highway Network, the MTO is not responsible for winter maintenance nor liable for damage incurred as a result of using these routes. The MTO introduced the Tertiary Road system in 1962. With the exception of Highway 802, none end in settlements.

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