It's interesting to find that despite the Liberals saying twinning the tunnel was never a serious option, an FOI request found that
they met with a Dutch engineering company who prepared a report on twinning the tunnel. This was found after an initial FOI request (done when the Liberals were in power) didn't turn anything up, and now that the NDP are in power they somehow managed to find the report.
Judging from the summaries I've read twinning the tunnel does in fact tick all of the same boxes as a 10-lane bridge would, except for being able to transport hazardous materials through it. It increases capacity, it's cheaper, it would be done so that the river depth could be increased, it's actually safer than an open highway, it has a smaller footprint and is less damaging to the environment, and it can accommodate foot and bicycle traffic.
This review from 2016 seems to have the opposite views as safety of a tunnel from the Dutch engineering report. It almost seems like the Liberals wanted to hide the Dutch engineering report because it didn't fit their narrative.
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