|alt=A two-lane undivided road in a rural area with a reassurance marker for Ontario Highway 48 north pointing straight Highway 48 incorporates a significant portion of the former Scarborough and Markham Plank Road, now known as
Markham Road, into its length. This section was not incorporated into the highway until 1954, yet predates the Highway 48 designation entirely. Markham Road began as the eighth concession east of
Yonge Street in the
Home District of
Upper Canada, and was
blazed by settlers to whom land had been granted along the
right-of-way. The right-of-way extended from Lake Ontario in the south to what is today
York Region Road 8A (Baseline Road) in Sutton, just south of Lake Simcoe, in the north. Improvements to the road and the necessary funds were authorized by an act of the Upper Canada provincial parliament on February 13, 1833 for the section in Scarborough township between
Danforth Road (present day Painted Post Drive) and the Eighth Concession at the border with Markham township. These improvements were supervised by residents Peter Secor, Richard Houck and Robert Armstrong. By 1847, the section between Scarborough and Markham had become known as the
Scarborough and Markham Road. On July 28 of that year, the parliament of the
Province of Canada passed an act to establish the Scarborough and Markham Plank-road Company, which was authorized to further improve the road surface to
macadamized or
planked construction between
Kingston Road in Scarborough and Markham Village in the north, and further north and then east to Stouffville along the Markham-Stouffville township line, it was paved in 1947. On February 10, 1954, the highway designation was extended south to the future site of Highway 401 in
Scarborough — though not all the way to the then-
Highway 2 (Kingston Road) — where a
cloverleaf interchange was constructed in anticipation of it developing into a freeway around the eastern side of Lake Simcoe; Highway 404 was constructed for this purpose, but along or parallel to Woodbine Avenue instead. In 1962, the highway was extended to Highway 46 at Bolsover via a concurrency with Highway 12 north from Beaverton. This routing would last until November 4, 1966, when the Beaverton Bypass opened, routing Highway 12 to the east. A new road was opened connecting Highway 48 south of Port Bolster with the bypass on the same day, and both Highway 12 and Highway 48 were rerouted. Portions of the former route of Highway 48 and Highway 12 were renumbered as Highway 48B. However, the segment between Port Bolster and what is now Brock Sideline 17 was decommissioned entirely. |alt=A narrow abandoned road in a wooded area with a two-lane road visible to the right On June 28, 1967, the routing of Highway 46 was shifted in the vicinity of
Balsam Lake on to a new inland bypass; the old route became known as West Bay Drive. On 1975, Highway 46 was truncated at Bolsover; the severed section was renumbered as an extension of Highway 48, bringing it to its peak length of . The new section of highway between Highway 12 and Bolsover was reconstructed over the following year, opening to traffic on August 19, 1976. The section between Highway 401 and the then-unopened
Highway 407 interchange was turned over to the Region of York and the City of Toronto on April 1, 1995, and is known as Markham Road south of Highway 407, and Main Street thereafter to Sixteenth Avenue, where the name Markham Road resumes for until Major Mackenzie Drive. The section within York Region is also designated as
York Regional Road 68. The section from Highway 12 to Highway 35 is now known as Portage Road and signed as Durham Regional Highway 48 and Kawartha Lakes Road 48. == Major intersections ==