http://www.wcsh6.com/news/article/272009/314/Cable-barriers-have-big-impact-in-preventing-serious-accidents
This is about the 7-mile segment of I-295 in Yarmouth, ME, heading north out of Portland. Reporter Tim Goff notes how they can't use them in many curvy areas. He stands by the area of Exit 8 (ME 26 - Washington Avenue) in his piece.
Cable barriers are definitely for straight stretches of road, since they're redirective by nature with a lot of "give." If you punch one of those things at anything above a shallow angle, you're going through.
3.5 mi northbound + 3.5 mi southbound = 7 mile stretch.
That figure still seems a little high... Maybe they included ramps.