"Not a good idea then".....depends on why you want to jump from the bridge. If you're comtemplating suicide then it IS a good idea, if not then why do want to jump off the bridge. The other factor in "chances for surviving" is if you are able to swim or not.
From that height your velocity will make the water roughly equivalent to hitting concrete. unlikely to survive, so if you are contemplating "tombstoning" you would indeed need one!
I drive over that bridge (mostly at snails pace due to traffic) every single day. Sometimes, i just fancy getting out the car and jumping off (for a laugh). I'll now have to reconsider my stress relief techniques.
So... from what height could a person jump into water without certain death? I've heard stories of people surviving falls from aeroplanes! At what point does water have the same effect as concrete?