Drug companies in Britain experimented by putting pills in blister packs instead of bottles and found out that suicide rates dropped dramatically. Apparently, the amount of time it took to pull each individual pill out of the blister pack was enough to make people think about what they're doing.
The same thing applies with bridges. There have been many efforts to put signs up and obstructive railings in place. They won't deter a really determined person, but they do the next best thing - the people who aren't sure are shown that it's a decision rather than a done deal. And that's what they need at that moment in time.
A short transcript[1] from the podcast agrees though, and there's a currently previous[2] and ongoing[3] research into packaging/impulsivity.
There's some other research as well that shows that restricting pack size of OTC paracetamol/acetaminophen reduces suicides, too[4]
[1] http://www.freakonomics.com/2011/05/17/why-cant-you-buy-a-bi...
[2] http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=ht...
[3] http://clinicaltrials.gov/ct2/show/NCT01118208
[4] http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fj...
They're not. This isn't a problem that you can't apply normal logic to. Instead you need to look at what works and copy that - and last-minute disruption/difficulty works.
Here is an example of a suicidal thought process, from my own experience: "I don't want to die, but given the situation I'm in I can't see any other option. I don't want to do it. I just need a reason to keep going."
When you can't find that reason, you do something fatal. The reason can be anything, the smallest thing can keep you going.
Some people kill themselves for different reasons, and I think people that have never suffered from depression have that kind of suicide in mind. The guy who kills himself rather than live with the shame of blah blah blah. (Or for the insurance money for his family.)
That's not a depression-related suicide, and I suspect they're pretty rare. Someone with depression isn't thinking that rationally. If they were, they wouldn't be suicidal in the first place.
This "they will find other ways" idea is silly, dangerous, and not supported by the facts.