I've crossed it a couple of dozen times, easily. Been going to that park since I was a little kid. It's always been a place that we would go for picnics and to walk around and take pictures. When I was younger, going across the bridge didn't bother me that much. (It always bothered the adults to watch us, ha ha, but as kids we were fearless.) I found that the older I got, the "scarier" it got. The last time I crossed it, which was a little over a year ago, I just happened to look UP (not down) and I completely lost my balance. It freaked me out. I liked to have not finished the crossing. Definitely not a normal pace or gait. Some of the slats are kind of far apart and I have small feet. It's more like a take one step, bring your feet together, take the next step.
There are parts of the bridge where you could see the little road underneath but it's not something you can see under the whole thing. If someone were standing under the bridge it would depend on where on the bridge that YOU were standing to determine whether or not you could see them. It is a very long bridge, much longer than it looks in the pictures. Like I said, some of the slats are kind of far apart but some are very close together. To get a good view you'd have to move over to the edge and kind of lean your body to one side. Not something I would do.
Some people speculated that Abby was looking down in her picture because she was scared or even looking at someone below her. I am not surprised that she was looking down because that's how most of us walk on that thing. You have to watch your footing.
Also, it hasn't been brought up here, I don't think, but there was some speculation that people were hanging out under the bridge-as in RIGHT under the bridge to where they could climb up over it and surprise the girls. That's not possible. There's really nowhere to stand and the height between the rails and the slats is just too great. I will eat my hat if we learn that BG had a companion dangling off the slats and waiting for the girls to finish crossing.
What I am surprised about is IF there was really a couple there standing under the bridge at around the time the girls were there, they didn't see or hear the girls. Because while it's difficult to see directly beneath you when you're ON the bridge, it's definitely easy to see/hear the people walking above you when you're UNDER it. I've been under the bridge a couple of times and we've always been immediately aware whenever someone started walking across it.