The bridge has been getting a lot of publicity over the years because of how many people jumped. I'm not sure about how the current fencing is right now, but it was pretty awful before. But there are constant patrols there for people looking to jump. Getting over SI can take quite a while, and no one's going to just ignore a stopped car in the middle of a busy bridge.
As for whether it'll be instant, it likely will be pretty fast. But the amount of pain you're likely to feel is uncertain. Some people swear that the adrenaline stops you from feeling anything, but others have said different things. Honestly, the issue is mainly a lack of evidence due to how it's impossible to test in a scientific way.