It is still possible for a large vehicle to turn left at Bush St in Belfountain after that sign, though that's legally problematic because of this:
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I'm not sure why there's a no-trucks sign for Bush St ... I smell NIMBY/politics.
It is legal, though a bad idea, for a heavy vehicle to take Mississauga Road north where it splits from FOTC, and the signage at that junction reflects that ... but that no-trucks sign on FOTC is easy to miss.
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In a more sensible world there would be signage in symbols (not words) on Mississauga Road approaching Bush St prohibiting truck traffic from going straight and requiring it to turn left on Bush St ... eff the local NIMBY residents at that point. Actual truck traffic that needs to go that general direction should have taken Olde Base Line and Winston Churchill or Highway 10.