History of Provincial Highway 12A:
Provincial Highway 12A was one of Ontario's earliest highways, extending from the Trans-Provincial Highway (Highway 2) at Port Hope northerly to Peterborough. The history of Highway 12A dates back to 1920, when the Department of Public Highways of Ontario (DPHO) assumed ownership of the Port Hope to Peterborough Road. A Preliminary Route Plan was prepared dated July 8, 1920, showing the proposed route of the new Provincial Highway through Northumberland County, while a second Preliminary Route Plan dated July 12, 1920 showed the proposed route through Peterborough County. The entire highway was assumed by the DPHO on August 11, 1920. The sections of the road passing through Port Hope and Peterborough were not assumed by the DPHO, and thus those sections of the route remained under municipal jurisdiction. Until 1925, the highway was simply known as the Port Hope-Peterborough Highway. The route was assigned the designation Provincial Highway 12A when route numbers were first introduced on Ontario's Highways in 1925. Strangely, Highway 12A did not actually intersect with Highway 12 along its entire 47 km route.
Most of Highway 12A was gravel-surfaced at first. In 1926, just over 5 miles of Highway 12A was paved with a macadam surface between Bailieboro and Fraserville. Highway 12A was paved with a sealed macadam surface for 10 miles between Fraserville and Peterborough in 1927. Just over 3 miles of concrete pavement was laid south of Bewdley in 1927, with an additional 7 miles of concrete pavement laid on Highway 12A north of Port Hope in 1928. At the time of the route's renumbering in 1928, the only remaining gravel section on Highway 12A was between Bewdley and Bailieboro.
During 1928, the entire route of Highway 12A was renumbered as Highway 28. This made the Highway 12A designation the very first route number to ever be permanently retired from the provincial highway system. Provincial Highway 12A was one of only two highways in Ontario that came into existence and disappeared altogether prior to the changeover to the King's Highway designation in 1930. Highway 12A and the other route, Highway 4A, are thus the only highways in Ontario that were solely known as a "Provincial Highway" and were not later designated as a "King's Highway".