Hmm. Perhaps we will see 404 going all the way up to Highway 11 one day. Though it's down to 14,400 once it's through Orillia (and Atherley).
12 on the east side of Lake Simcoe is busy, but nothing crazy - it gets significantly busier on summer weekends but that's like 8-10 weekends a year. For the other 350 days a year there are a lot of other busier highways around.
Through Orillia it's a lot of urban traffic for sure - it just needs a more traditional 4-lane arterial configuration, IMO.
For those curious, the other really busy 2-lane provincial highways are:
1. Highway 6 through Morriston - 29,000 AADT
2. Highway 12 through Orillia - 26,000 AADT
3. HIghway 7 from Kitchener to Guelph - 22,000 AADT
4. Highway 9 from Hwy 400 to Orangeville - 22,000 AADT east of Schomberg and 15-20,000 west of it
5, Highway 89 from Alliston to Hwy 400 - 18,000 AADT
6. Highway 5 from Waterdown to Sydenham Rd (Dundas) - 17,000 AADT
7. Highway 6 south of Fergus - 15,000 AADT
8. Highway 10 from north of Orangeville to Shelbourne - 15,000 AADT
All other highways operate under 15,000 AADT which is generally where roads start to experience significant congestion. A couple others come close, like 6 between Caledonia and Hagersville and 12 around Beaverton, but aren't there yet.