I'd imagine the size of the bridge itself might be a factor. The two in question in Vancouver (Alex Fraser/Port Mann) would dwarf the Zakim bridge (that's what

I'd imagine the size of the bridge itself might be a factor. The two in question in Vancouver (Alex Fraser/Port Mann) would dwarf the Zakim bridge (that's what I'm assuming you are talking about). I suppose an engineer would have more input on this though, I really don't know too much about bridge design.