not sure what this had to do the parent comment, but if there's a net, people will find other ways. It's like making prostitution illegal, did it really cut down on the number of people having sex? Of course not.
As said by Scott, suicide is very rarely a well thought-out action. It's something that is decided on a whim and often passes just as quickly.
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.
I'd never considered blister packs as an impulse control measure before; I had assumed it was for ease of dosing/packaging, and perhaps longer shelf-life (The US approach of (until relatively recent automation improvements) manually counting pills into a bottle always struck me as inefficient).
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]
But this effectively removes the chance that it'll kill you. Getting to the GG Bridge is not a simple thing to do... If you're going to kill yourself, the impulse is there, sure, but you had to go pretty far out of your way to do it. With a bottle of pills, many people have a bottle of pills from a previous surgery or something sitting around. Way easier to get wasted while depressed and make a rash decision to down the bottle than it is to make the trek all the way out to the middle of the Golden Gate.
You're making the mistake of assuming that someone who is going to end their life is thinking in any kind of rational way.
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.
But removing the ability to kill oneself by installing a net removes the possibility. If you're going to kill yourself, you don't try to do it via a means that has no chance of success. With a net installed, all the chance of success is removed, so people aren't going to try it there. You don't hear about failed suicide attempts jumping off a car onto grass. And I've battled with severe depression and alcoholism in my past. I spent nearly half a year thinking I was going to end my life every day and instead crawled into a bottle. You're right in that we're not thinking rationally, but at the same time, irrational "I must escape" thoughts also don't drive you to say "hey, I know, let me go way out of my way to attempt suicide in a way that I know will not work."
edit: if anything, more people will probably jump, but not to kill themselves. thrillseekers might do it just for the hell of it (despite the fact there will likely be a big penalty)
From what I have heard and read (see the Freakanomics podcast I link to in this thread), suicide is seen among those that study it as having a large impulse component.
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.