https://www.youtube.com/watch?v=r4aYxObfjJ8
https://en.wikipedia.org/wiki/Port_Mann_Bridge#Original_bridge The original PMB had only 2 lanes each way with no emergency lanes or wide shoulders. It was designed to be a classic BC bottleneck-chokepoint right from the start. Eventually, a 5th lane was squeezed in.
https://en.wikipedia.org/wiki/Port_Mann_Bridge#Opposition_to_twinning_plan While bridge duplication isn't that big of a problem in Australia or the US, it is in the BC part of Canada. Australia is allowed to have 3 proper big cities on the Pacific. Thus, the urban scale of infrastructure in Melbourne, Sydney and Brisbane are much larger than what's allowed in the Greater Vancouver Region.
https://en.wikipedia.org/wiki/Port_Mann_Bridge#New_bridge Given that this is supposed to be part of the main East-West highway in Canada, a significantly wider bridge was eventually approved. While it was designed with a provision for a potential future rail line, there should have also been a provision for a lower deck.
