You pose two good questions. The issue with the future development of W-470, is a plus, in my opinion. It allows better access to those areas you are considering, and I view that as a benefit. The highway isn't there yet, and based on the past experience, it may be many a year before we see that in place. I would just make sure that the lot I chose to build on was far enough away from the proposed highway, that you wouldn't have a road/noise stigma. Location is everything, they say!
Your second question about Rocky Flats, is a little more complex. I am not a scientist and cannot tell you what residual effects from the use of radioactive materials still may exist in those lands surrounding the original Rocky Flats plant. Having turned that land into a wildlife preserve may say something about what is thought of the continuing hazard(if any). If you're concerned about the presence of radioactive contamination, I guess the best way to determine that would be to have someone test the area, and specifically the lot you were considering building on. The issue of Rocky Flats and its impact on the surrounding environment has raged the whole time I have been a Realtor, but I'm not sure that issue is as much a concern today, as it was 35 years ago.