The only real merit of the tunnel that I could really see is that it would be better protect people from the elements whereas a bridge (unless covered on all sides) leaves you open to wind gusts, snow and rain. But apart from that, tunnels are a strange experience and don't make people feel that safe, even if they are. There is a psychological aspect to them that makes people tend to avoid using them. This is especially true for women who could easily experience sexual assault or rape inside of something where nobody can see but it's also much easier to rob someone inside of them. A good sucker punch from behind and someone can grab your wallet and phone then run off. Bucket Man often sits around the tunnels under Westmout Road (near Highland Road) and has assaulted people for no reason whatsoever. With a bridge, perhaps people don't have that good of a line of sight either but they're still much more exposed to anyone walking on it or who could view it from neighbouring buildings so they are both tangibly and psychologically safer experiences.
You would also need to spend more money on tunnel maintenance (light replacement, graffiti removal, cleaning...they always smell like piss, collect trash/leaves etc). It would also need to have adequate drainage to deal with rainfall and snowmelt that can pool up inside of it, driving up the costs even more...and that can be a tricky thing to achieve if the tunnel is deeper than the storm drain systems running under the streets.
You would also need to spend more money on tunnel maintenance (light replacement, graffiti removal, cleaning...they always smell like piss, collect trash/leaves etc). It would also need to have adequate drainage to deal with rainfall and snowmelt that can pool up inside of it, driving up the costs even more...and that can be a tricky thing to achieve if the tunnel is deeper than the storm drain systems running under the streets.