The articles governed by this project are subject to several guidelines. Some are set by the project and should be followed in order to maintain a consistent appearance across articles. Other, however, are set by the Wikipedia community, and the general consensus is that they be followed unless there is a good reason not to.
Mandatory guidelines
AA:MURA - The Manual of Style (MOS) governs the appearance of all articles on Wikipedia.
AA:RJL - The Road Junction List (RJL) guidelines is part of the MOS.
The MTO provides a list of current Connecting Link agreements as of 2020. This should be used in conjunction with the AADT data provided by the ministry, which will show Connecting Links by "CL" "C/L" or "NA" (though the last also refers to Non Assumed portions of highway), in addition to Google maps and the MTO centreline datasets.
Secondary highways designated at the beginning of 1956
"Ontario Secondary Roads Now Designated 500, 600". Vol. 112, no. 33, 119. The Globe and Mail. February 4, 1956. p. 4. Two new Ontario road numbers appear on the province's 1956 official road map which will be ready for distribution next week. The new numbers are the 500 and 600 series and designate hundreds of miles of secondary roads which are wholly maintained by the Highways Department. More than 100 secondary roads will have their own numbers and signs this year. All of these secondary roads were taken into the province's main highways system because they form important connecting links with the King's Highways
"Provincial Highways Now Being Numbered". The Canadian Engineer. Monetary Times Print. 49 (8): 246. August 25, 1925. Numbering of the various provincial highways in Ontario has been commenced by the Department of Public Highways. Resident engineers are now receiving metal numbers to be placed on poles along the provincial highways. These numbers will also be placed on poles throughout cities, towns and villages, and motorists should then have no trouble in finding their way in and out of urban municipalities. Road designations from "2" to "17" have already been alloted...
Template:Cite ONRD - Produces 8 common citations: MTO km log, 1989 km log, MapArt's Ontario mapbook, DHO/MOTAC/MTO annual reports, 1997 highway transfer list, 1998 highway transfer list and 2011 Connecting Link list
Other sources
You likely won't find everything on the internet. There are a few important places worth checking if the resources are not digitized:
Local historical societies are often receptive to questions or research inquiries on local roads, even by email. Let them know you're writing for Wikipedia and what specifically you are looking for. You may get newspapers or books that are otherwise unavailable rather quickly.
Local libraries will have engineering plans for local project. But more importantly, a membership often provides access to digital resources. ProQuest is often the provider of these, and if your library provides it, the Canadian Newsstream database has all available Canadian newspapers from ProQuest.
Globe and Mail (libraries)
Toronto Star (libraries)
Ottawa Citizen (libraries, Newspapers.com - Publishers Extra after 1923)