I cycle through Japan and a few days ago I drove the Shimanami Kaido, a series of bridges connecting islands between the mainland and Shikoku in the inland sea. One of the bridges there was (if I remember correctly) 6.5 km long. This had a custom bike / pedestrian / moped section.
On Route 317 between Matsuyama and Imabari, I also came across several tunnels, the longest of which was 2.8 km long. There was not a specific bike path per se, but on one side there was a sufficiently large elevated road so I was completely safe (that is, there were areas where the lights had blown and I was driving in jet black!).
That made me think. What are the longest tunnels and bridges? that's for sure for cyclists to cross / pass? I will add the additional condition that it does not have to be specific illegal,