Ya right now the best bang for your buck is to widen the bottle-necked stretches. 427 to 409 is top priority for sure, but there are also other weird lane drops in places like the WB collectors at Yonge where you only have 2 lanes to Hogg's Hollow bridge. I'm not saying a entirely new bridge is needed to fix that (although it would help with repair/maintenance if one bridge could be completely shut), but a rework of that interchange could see 3 collectors pushed through.
-You could also simply take out the Yonge to 401 WB ramp entirely and foce traffic to the loop ramp with a left turn. That would not be good for traffic on Yonge but it gets the 401 WB 3 through lanes on the collectors.
Crude imagery of what I mean
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The Yonge Overpass would also need to be extended out a bit to fit the extra collector lane, or shoulder cannibalized. You could also reduce the radius of the loop and build a new parallel bridge, with merge room for the onramp between the Yonge overpass and Hogg's Hollow bridge.
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Under Allen there are also only 2 EB collector lanes running through. If you cannibalized the shoulder you might be able to fit 3 tight lanes under the Allen to 401 EB express ramp, or that flyover would need a rebuild.
The section I am referring to.
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Fixing these would give you a minimum of 3 collector and 3 express lanes throughout Toronto proper (along with adding them between 427 and 409).
Outside of Toronto limits, It's insanity that the lane drops between Salem and the 412 have not been fixed yet. You go from 5 each way down to 3 just before a major interchange... get those lost lanes extended a tiny bit so they become dedicated exit/entrance lanes for the (now toll free) 412. This is a stupid, easily fixable bottleneck that can snarl traffic for EB traffic for many KMs.
-I know the collector/express system is proposed to expanded to the 412, but this is would work in the interim.