I wouldn't want to feel pain while being broken and splattered on the floor. How high would one need to be for instant death? What are things that can get in the way of survival (i.e. wind/trees)?
I have been planning to go with jumping too. There's really no telling on pain, you just can't know if you'll die on impact, people tell to jump head first but it's really hard to control how you land. In regards of things that can get in your way try to choose a spot with not much vegetation and also pay attention if cars tend to go there, landing on a car could not only save your life but also injure other people.
Try to look for local newspaper articles of people killing themselves by jumping, they tend to write about those attempts often, that's how I've been looking for bridges that are eligible.