I don't think they made it all the way to the bridge. It looks to be at least 3/4 mile from the girls house. Plus, they were found on the right side of the road to the side, as if a car had just come up from behind them (or they were accustomed to staying near the edge of the road while walking just in case a car came up behind them). It looks like they were still walking toward the bridge, not away from it from the position of the bodies and the various pictures I've seen of the memorials.
I read one article that says the 911 call from the grandfather was at 5:41 p.m. and that he had walked the road to find the girls after they didn't answer the cell phone. I wish we knew when Skyla's mother called to say she was coming by to pick her up (which also doesn't make sense if the sleepover was Sunday evening, why pick her up that evening).
There was another interview where Skyla's mother says she felt something was wrong when she was driving up and saw Taylor's grandfather by the side of the road. So, did she arrive after the call, but before the authorities where there?
Anyway, from the reports, I don't think they made it to the bridge. I don't think they saw anything, and it makes it sound like they may have been killed between 5:10 and 5:20, closer to the start of the walk than the end of it.