bond1111 said "(post removed)"
I mean, you're right in that it may not necessarily deter people from committing suicide, simply displaces them to do it somewhere else. A quick Google search suggests that studies have been inconclusive. But it's important to remember that very often, suicidal thoughts can be very fleeting, and an impulsive person may act on them (and succeed) if there's an "easy" opportunity. They definitely reduce/eliminate suicides *at that location*. I've lived here 18 years now, and I can think of a handful of times that people have either tried or successfully jumped off either the 18th or 1st Street bridges.
Also, I've known people (or know people who know people) who drive busses, trains etc and have been impacted by someone choosing their vehicle to aid in their suicide. It's not just a bridge, but a bridge over a busy rail line. Their mental health is important too.
Plus, it stops people from throwing stuff off the bridge onto the train tracks.