I want to start this thread and have it in the list of public research. As I have time I will keep on adding stuff.
First off, in Canada, it is the responsibility of the province to plan, build and maintain their roads through the provincial ministry of transportation.
In Ontario it is called the MTO ( Ministry of Transportation of Ontario).
"The King's Highway" is the name of the road system.
In my Opinion on how the province should be represented regarding SCALING at 1:20 :
For ATS at a 1:20 scale, we don't want to over pack the DLC because we would loose the sense of distance between areas of the province. I recommend the main trans-provincial roads only with spurs to some cities / industries along the way.
Highway 11, 17, 400 / 69 and the 401 are the main roads that would make sense in an Ontario DLC.
The 402, the QEW (Queen Elizabeth Way), the 416 and the 417 could finish off the DLC nicely.
As for the Golden Horseshoe (Niagara to Durham via Hamilton), I recommend only the QEW up to the 427 and up to the 401 at the Pearson International Airport area where most of the warehouses are. There could be a bit of Dixie Road where there is a real Flying J truck stop (only one in the area at Dixie and Britannia) and many companies around there.
I also believe that it should be Mississauga to be marked as a city and not Toronto because yes Toronto has lots of industries but I feel that the industries in Toronto are all over the place mixed within residential areas and trying to represent this at this scale would ruined the game experience, while having a certain hub around Dixie Road could make it to represent the Toronto area (this is my opinion);
in Toronto itself I only see the junction with the 401 / 400 and a somewhat representation of the collector / express system (but again at 1:20 it would be a very short representation... so maybe not)...
I don't recommend modeling the Gardiner Expressway and the Don Valley Parkway, again because of scaling issue BUT I will say that the downtown area offers Redpath Sugar and the port of Toronto (industries possible there and downtown outlook as well for a SCS viewpoint!!!).
I believe that the only main interchange within Toronto City would be the 401 @ 400 because the 400 (with a piece of the 69 which is by the way being twinned and that will become the 400 eventually) is the only expressway that links South Central Ontario to Sudbury (the door to Northern Ontario) and connect to Manitoba in Canadian Land only.
As for Durham, it would be good to implement the General Motors Oshawa Assembly Plant and the Darlington Nuclear Power plant.
My opinion on The "407 ETR":
In Ontario, we have one inter-regional major expressway owned and maintained by a private company. It is a tolled road with a state of the art toll collection system using cameras and your license plate; there are no booths (you receive the bill in your mail at the address linked to your license plate or you can also register with a transponder and pay the bill online).
407 ETR (express toll road) is its name. Vehicles having a Gross Vehicle Weight Rating of 5000 Kilograms and more driving on this road must have a transponder (yes, even if you don't have one, you still can go on the road, there are no physical barriers, but when the camera and employees find out, you will get a big fine on the bill.
In ATS, I am not sure that the 407 ETR is relevant because it is fairly close to the 401 and because the road is owned by a private company maybe there could be some licensing hurdle... however the latest eastern portion of the 407 is still a toll road but owned by the MTO as well as the 412 and the 418, but again too close to the 401, might as well ditch the 407 as a whole... (just my opinion)...
Others:
Right hand turns on a red light are permitted throughout the province.
Right hand turns on a red light are not permitted where prohibition signs are in place on the light pole (very rare though).
Ontario King's Highway Reassurance Shields example:

Ontario King's Highway Advance Approach Shields example:

work in progress, more to add