Photo from commons.wikimedia.org
Steepest kilometer begins at km 8.8 (12.7%)
Sign at mile 3.4 says "prohibition of transit unless authorized" - sign also says private road.
We will document this climb in August, 2018. Our reserach indicates the road is paved, but a serious challenge. 360 degree streetviews at locations up the climb taken by private citizens (not Google) show cement or pavement on the road, including hte last stretch to the dam. Strava members have stated, in response to questions (a) is it paved all the way and (b) can it be done on a road bike:
Sergio Gianola: The segment is practically all paved and can be done on road bikes, the last mile has two short pavement tracts - The segment is virtually all asphalted and can be done on road bike, the last kilometer has two short sections of pavé
Fabio Rinaldi: yes i did it
Pozzo Ciao: certamente, of course. Sometimes there are dangerous manholes that you must go across diagonally
Mattia Pozzo: Last 400mt has a Stone and cement surface and it's about 16%