The Idaho Falls Greenbelt follows both sides of the Snake River through downtown Idaho Falls. If you stick to either the north side of Broadway or the south side of Broadway you can run for several miles without ever having to cross a road or worry about vehicle traffic. The section between Pancheri road and Sunnyside road is less popular and goes through some unmaintained trails and through South Tourist Park. I've come across homeless people hanging out/camping/etc. in both areas, which doesn't bother me, but some people would want to take a friend with them if they are running that part of the trail. Everything north of Pancheri road, feels much safer and there are typically decent numbers of other runners on the trails.