Longest bridge/tunnel that can be cycled over/through?

The longest tunnel open to cyclists in Japan is the Kanpu Tunnel (not far from the Route 317 tunnel listed in the question) and is 5,432m long. It has a reasonable gradient and took me the best part of 45 minutes to get through on my loaded touring bike. I rode through in the early morning and it was pretty much empty of traffic so it is definitely safe (especially if one goes the downhill way).

Japanese drivers are very courteous to cyclists (even if they pass a bit closer that you might be used to) and there are many tunnels that are many kilometers long that cyclists can get through safely.

Additional bonus answer:

In Germany there is a mine “Erlebnis Bergwerk Merkers” 800m in the underground, which performs regularly bicycle events.

For example on 31.03.2019 there drove 50 cyclists 15km from “Schacht III” (lifting hole 3) to the “Kristallgrotte” (crystal grotto) and back.

Some tours are only cycling, others have stations where you could learn about the mine.

(Cycling tours in German language)

(Mine Merkers in general in English language)

Another “bonus” answer, longest elevated bike path 7.6 km (can be considered a bridge in some sense?). In the city of Xiamen, China.

https://youtu.be/hUbQyNLkjvo

Not the longest, but the 9 km long Oosterscheldekering is certainly noteworthy. Location of the Dutch Headwind Cycling Championships. This is the longest storm surge barrier and flood barrier in the world. There’s a road and a seperate cyclepath crossing it. It consists of 65 pillars of 30 to 40 meters tall, with 62 sluice-gates of 40 meters wide and 6 to 12 meters tall over 3000 meters. It is also a hydro power plant. This is listed as one of the seven wonders of the modern world by the American Society of Civil Engineers, along with the likes of the Golden Gate Bridge and the Panama Canal.

While you’re there, you’d also have the opportunity to cycle on/in:

• The 5 km (3.1 mi) long Zeelandbrug, which was the longest bridge in Europe from 1965 to 1972.
• The the 6.6 km (4.1 mi) long Westerscheldetunnel in the Netherlands. While usually only cars are permitted to drive through it, a few days a year it is open for cycle tours. The next one going through is the ZLM Tour on the 20th of June 2019.
• The 10.5 km (6.5 mi) long Oesterdam. Part of the Delta Works, just like the Oosterscheldekering. This is a landbrige. Similar to the Afsluitdijk mentioned in gerrit’s answer.

The Chesapeake Bay Bridge Tunnel is 17.6 miles (28.3 km). It’s not normally open to bicycle traffic (though shuttle busses are available to take cyclists and their cycles), but at least twice in its history, one span of the bridge has been opened for special cycling events (with cars redirected/restricted to the other span). If this qualifies, it’s almost certainly the longest.

The longest bridge one has already been covered, but the Hiawatha trail in northern Idaho features the St. Paul Pass Tunnel which is 1.661 miles (2.673km) long and it’s designated specifically for bicycles. https://www.ridethehiawatha.com/the-trail

Most of the world’s longest road tunnels are on express- or motorways, where cycling is prohibited anyway, independent of the tunnel.

The longest road tunnel I could find, which is definitely open for bicyclists is the 8,079m long Steigen Tunnel in Norway.

I would however also assume that bicycling is allowed in the 14,346m long Mount Ovit Tunnel in Turkey. I can’t find any specific information about bicycling in this tunnel, but the road is designated as a regular highway (not a motorway) and bicycling is generally allowed on Turkish highways, even on 4-lane highways.

If it is safe or not is a matter of taste. In any tunnel, even very short tunnels, passing vehicles may cause severe and difficult to predict draughts, which can be tedious to cope with when bicycling. Most longer tunnels also have a cold, moist and uncomfortable climate.

Completely safe alternatives, but not nearly as long, are all the tunnels along the decommissioned railway lines, which have been rebuilt as pedestrian or bicycle paths in many countries over the past few decades. In Europe, the 2,630m long Uitzi Tunnel in Spain is probably the longest. I am not sure if it is currently open, as it has been frequently closed during the last years for maintenance or because of water leaks.

Not a bridge or a tunnel, but as a bonus, consider the Afsluitdijk, a 32 km artificial enclosure dam and causeway connecting the Dutch provinces Noord-Holland and Friesland, separating IJsselmeer from Waddenzee. As a causeway, I’d argue it’s somewhat similar to bridges and tunnels, in that it connects two landmasses otherwise separated by water. Enjoy cycling it, in particular with a strong headwind.

Source: C.S. Brooms/Kris Rodenburg, Rijksdienst voor het Cultureel Erfgoed, via Wikimedia Commons

The Seven Mile Bridge in the Florida Keys is open to cyclists. The Florida Keys Overseas Heritage Trail, a nearly-complete 106 mile (171 km) cycling path from Key West to Key Largo, and part of the East Coast Greenway which stretches to the Maine-Canada border, is routed over the bridge. (The bridge is actually 10.9 km or 6.8 miles long.)