Way back in the 60s and early 70s at the height of our auto-centric civilization, there were plans to build DVP like expressways down Spadina and Keele to connect the downtown with the 401. As they were both being built, opposition started to spring up, and it was largely the Annex residents like Jane Jacobs and John Sewell who in stopping the Spadina expressway also stopped the one slated for Keele and Parkside Dr. The legacy of that era is both the Allen Rd and Black Creek Drive mini-highways that stop suddenly at or just past Eglinton.
I believe the timing of the Weston Road Bridge coincides with this planned highway development. Rather than repair it they assumed it would be replaced by the new expressway. When that ended so did traffic planning in that part of the city.
The intersection of Dupont/Dundas/Annette even with new lane configurations and paint is still a crazy zone. Following the link to the photos on flickr you get an anecdote about junction rd and old weston road and how there's always traffic there. That north bit of old weston road and davenport is also a crazy traffic zone as traffic increases to reach the big box stores at keele and st. clair.
The good news of course is the slow but steady decline of the auto-centric civilization, and with the new mural we're starting to see this old rail area get reclaimed by bikes and pedestrians, with autos confined to single lane. Although east on Dupont there's now parking in bike lane's which makes it dangerous re: getting doored or swiped...