Most of the bigger dams in the Alps have been built with railways and cable cars rather than roads, even when a small road was available. Of the two recent Swiss projects, Linth-Limmern has been built by using a funicular and a cable car to bring supplies to an isolated road network, while for the Nant de Drance (Emosson reservoirs) a 5 km road tunnel has been built.
One of the Linth-Limmern tunnels is open to tourists:
https://map.geo.admin.ch/?lang=it&topic ... 88&zoom=10 At 3 km, it is the longest pedestrian tunnel in Switzerland. During the construction of the first reservoirs in the area it was used by trains and road vehicles (which were brought there by cable car). Some second-hand Zürich's city tramways were also used, to transport workers.
Robiei is accessible only by cable car (
https://www.seilbahninventar.ch/objekt. ... 95&lang=it), which is open to the public (
https://www.fahrplanfelder.ch/fileadmin ... 1/2627.pdf). One vehicle of the cable car is a normal passenger cabin, the other is a sort of crane under which everything can be attached. At slightly more than 4 km, it is one of the longest single section cable cars. There is an endless number of cable cars in the Alps, most of them small and unfortunately closed to the public (often, when the project was completed, bigger cable cars were replaced with smaller ones used to transprot maintenance workers and lighter equipment even in winter).
I'm not sure the road to Narèt reservoir is still open to the public.
From time to time local politicians speak about a road, tunnel or cable car from somewhere near Airolo to somewhere near these lakes, but it always ends in nothing.