I'm cycling around Japan, and a few days ago I cycled 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 recall correctly) 6.5km long. This had a specially made bicycle/pedestrian/moped section.
I also have come across several tunnels, the longest of which was 2.8km, on route 317 on a mountain pass between Matsuyama and Imabari. There wasn't a specific cycle path there per se, but there was a sufficiently large raised path on one side so that I was perfectly safe (that said, there were areas where the lights had blown and I was cycling in pitch black!).
This got me thinking. What are the longest tunnels and bridges that are safe for cyclists to cross/pass through? I'll add the extra condition that it must not be specifically illegal.