OK, where to begin...
First, a look down perspective is OK, but when the look down is done with motion it better be done sparingly or some of your viewers will get nauseous -- I'd be one of them. In you 4+ minute video about 75% of it was look down with motion -- don't do that, please! Another problem with looking down with motion, particularly if relatively close, is that you make it much harder for the compressor to compress the video resulting in more blurring. You see very little detail in the rail lines and ties due to that motion. Also, a bit more than half way through the video, when you finally turn your camera up and travel the length of the bridge again, you did so on the shadow side of the bridge so that you do not see it as well as the sunny side. If you had made the pass on the other side the bridge would have been lit better and you would not be looking towards the Sun. As to how long to make the video, well, many here argue that less is more and although I often disagree with that I'd have to say that in this case there wasn't much added after the first pass of the bridge.
Brian