Ontario Road Map – Road map collector. Site by Neal.
The King’s Highway – The history of Ontario’s King’s highways. Site by Cameron Bevers.
Ontario Highways – Site by Christopher J. Bessert, Cartographer, GIS Specialist, Highway Historian.
The Ontario Highwayman – Site by Chris Beach.
The King’s Highway Ends Site – Old site by Earl Andrew Washburn.
Ontario Road and Highways – Yahoo group (active).
Asphalt Planet – Ontario, Quebec and US road history and photographs. Site by Scott Steeves.
Historic Roads – Dedicated to the identification, preservation and management of historic roads. Site by Paul Daniel Marriott & Associates. Washington, DC.
Historic Bridges – North American bridges. (Ontario link). Site by Nathan Holth.
misc.transport.roads – Google Groups.
Have you found a lost road and photographed it?
They aren’t that tough to come across. Read local history to find where routes were changed. Not every road grew into a bigger road, some were bypassed and forgotten. Those are the old roads to look for, or to start with. As you find old roads you will soon find other old and forgotten roads. Bridges too.
What about dead ends, do they count as a lost road or not?
Meanwhile, the links above will get you started with your own research and exploring.
Information for Highway Explorers
I found some US highway history. Likely the information will be similar for Ontario and Canadian highways and roads but… that will be another post. So far I found a lot of Canadian (and Ontario specific) resources but I haven’t done the research yet.
Prior to the Federal Interstate Highway system, the United States was criss-crossed by roads built by for profit groups. During the 1920s many of these roads could barely be called roads as they were more mud, dirt and ditches than road. But, as Henry Ford continued to churn out automobiles, more and more of these state highways popped up across the landscape. Most of these roads followed old trails or Transcontinental Trails like the Oregon and Santa Fe. One of the first transcontinental highways was the Lincoln Highway from New York to San Francisco. It was a rock road and privately financed; Henry Ford wanted nothing to do with it because he thought roads and highways should be funded by the government. As the 1920s progress other groups formed to build and promote their own highways. By 1925, there were over 250 named highways, each with their own colored signs, names, and random sign placement. Without government oversight, many of these roads were re-routed into cities so that the clubs and groups that built them could profit from them.
In the midst of this chaos, the Federal government got involved in 1924 and started numbering all of these roads. Odd numbers ran North to South with the numbers increasing from East to West, and Even numbers run East to West with the numbers increasing from North to South. So, U.S. Route 1 runs along the Eastern Seaboard while U.S. Route 10 runs along the Canadian border.
When the Interstate Highways came along, the government decided to use the mirror image of the numbering system to avoid any confusion. Interstate 10 runs through the southern states while I-5 is in California. Thankfully, the government was wise enough to help avoid the classic “How could you get us lost?” fight between drivers and map readers. Where the two systems, the routes and the Interstates, meet in the middle of the country it was decided that there would be no Interstate 50 to avoid confusion with U.S. Route 50 which runs from Sacramento, CA to Ocean City, MD. This is the same for Interstate 60.
via – 9 Things You May Not Know About the U.S. Interstate Highways | From The Kitchen Cabinet
When the Interstate Highway Act was passed, most Americans thought it was a good idea. But when construction started and people, especially in urban areas, were displaced and communities cut in half, some started to revolt. In the 1960s, activists stopped construction on highways in New York, Baltimore, Washington, D.C., and New Orleans, which resulted in several urban interstates becoming roads to nowhere.
The red, white, and blue shields used to designate interstate numbers are trademarked by the American Association of State Highway Officials. The original design for the shield was drawn by senior traffic engineer Richard Oliver of Texas and selected out of 100 entries in a national design competition in 1957.
A major concern during Eisenhower’s presidency was what the country would do in the event of a nuclear attack. One of the justifications for the building of the interstate system was its ability to evacuate citizens of major cities if necessary.
via – 10 Things You Might Not Know About the U.S. Interstate System | Mental Floss
One casualty of this cost-cutting mechanism was King’s Highway 2, which was the main east-west trunk through southern Ontario before Highway 401 was completed decades ago. 
I was at a farm auction once in 1995 and I saw a group of Ontario highway signs for sale. They looked brand new, but they were a configuration I’d never seen before, with number 3 on it. That made sense, since Highway 3 is nearby, but these were very different signs. I was a student at the time, and I wish to this day I could have afforded the $40 for one. According to my information, Highway 3 (which runs from Windsor to Fort Erie) remains largely intact.