Exceptional section of road and I want to go back there now. Everything the other guys have written is spot on.
The scenery on this route is remarkable. As stated previously it is tricky to find the right road out of Cernay, but well worth the effort to find it. The twists and turns make for a great ride. Watch out for the hardy souls who push bike up here...
Had the pleasure of riding this route last week and well worth the effort. D431 is not well signposted in Cernay and we had difficulty finding the right road. Lots of hairpins and climbing and busy at weekends with a right mix of bikers, Sunday afternoon drivers, cyclists and walkers - bit like the Peak District except more mountainous. Agree with the comment earlier - a few corners are cobbled and I imagine this would be nasty in the wet. Still, the views are stunning, the road surface, in the main is good, and Id recommend this as a great biking experience.
Good hairpins running up the hill to Viel Armand (A french war memorial cemetary on the top of a mountain), although there are about 3 hairpins which change to cobblestone surface rather oddly (could be very dangerous in the wet). The descent can be a bit tricky because of paint on road and bumps so be careful