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I am planning a long trip in Japan (about 1400 km) in a hilly/mountain region. I already know there will be no cycling path and then I will have to share the road with cars. This is allowed by local regulation.

Along the way there will be for sure some tunnels, and besides taking care of being overly visible, I wonder if it is safe cycling there. Since already crossing a tunnel within a car one can smell the exausts of the other car/trucks, what could be the effect of crossing it at a slower speed and with an higher breath-in breath-out pace?

Should I, when possible, consider alternate routes or I am being afraid of a storm in a tea cup?

EDIT The lenght of the tunnels can span from few hundred meters to 3-4 km.

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    I doubt it's any worse than what many urban commuters put up with daily. A few choose to wear masks, but most don't. And you'll only experience it for a few minutes at a time with breaks in between. (not an answer as no hard numbers on pollution levels, and assumptions about the tunnels) – Chris H Jan 5 '17 at 6:53
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    edited answer your comment – L.Dutch - Reinstate Monica Jan 5 '17 at 8:43
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    This article has some information about optimal walking and cycling speeds to reduce air pollution inhalation: sciencedaily.com/releases/2016/10/161027123002.htm This has information about tunnels in Japan: bikept.com/touring-and-travel/… If pollution is a big concern in tunnels you could get a face mask. – KeithWM Jan 5 '17 at 9:59
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    To judge by Google streetview, away from the expressway, you're unlikely even to encounter a single other vehicle!! – Strawberry Jan 6 '17 at 11:29
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    added in the text – L.Dutch - Reinstate Monica Jan 9 '17 at 5:50
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Will you be in Hokkaido?

We toured there extensively this summer and cycled through several dozen tunnels: here our findings. They might apply to other parts of Japan.

  • Newest tunnels on big roads have wide side pavements where you can (and possibly are supposed to) cycle. Safety is not an issue but you might have to dismount to make it on the pavement and it might be only on one side of the tunnel. Lighting is not an issue, we found them very bright.

  • Slightly older or smaller tunnels still on big roads can have a smaller side pavement, sometime with a fence/chain separating it from the cars. It might be higher than the road or level. Safety is not an issue but if you have a wide load steering can be a bit tight. Visibility can be bad. Bring strong headlights.

  • Old tunnels on big roads can have a very narrow pavement where you can still push your bike, or you will have to share with cars. You will have to make the decision before getting in, as you might have a chain preventing you to switch. We found just one of those in Hokkaido (Otaru) and while safe, it was unpleasant. Visibility was poor. Surface was mud and sand.

  • Tunnels on secondary roads you will have to share with cars.

Things to keep in mind: you could have locals riding the other way on a narrow pavement without lights, often peasant/fishermen who are hopping across for their errands. Or pedestrians. Be very careful as you might see them at the last moment.

We also met several roadworks in tunnels, and a few times on the pavement, with the chain preventing you to get back on the road, but the crews were all extra kind and we sorted it out.

In costal/heavy rain areas you will find sand/mud banks at the entrance of the tunnel or sometimes in some points inside. Not necessarily visible and affecting your steering.

Concerning heavy breathing, tunnels (with one notable 10% exception on a side road on a dam) were never steeper than 6% and this helped. We did not find fumes to be extremely bad, and sometimes just cycling in towns on a slope at a traffic light, for instance, you'd get worst exhaust.

For planning tunnel location, length, and possible alternative routes, refer to "Touring Mapple" (which you should not ride in Japan without!)

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Japan has relatively modest vehicle emission regulations - certainly not to EU standards. The worst that I experienced is diesel soot from buses and trucks. Luckily, rural Japan has depopulated to the extent that you can generally route in a way to avoid traffic. As far as tunnels go...

For shorter tunnels, there's not much you can do except hold your breath if you're in a tunnel with a truck or bus. Or wait until the fumes dissipate.

Longer tunnels in Japan have forced ventilation. They should have sensors that detect the air quality and spin up turbines to force air through them if the quality is low.

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    Whether it be fumes or anything else you don't want to breathe in, holding your breath is a bad solution unless you know for sure it's going to be over in a seconds or so. After holding your breath, you start breathing in deeper, making up for any savings while you held it.This is especially so if you're performing a physical activity such as cycling. I'd say: don't hold your breath, breathe normally, just try to get to the other end fast. – Michał Kosmulski Jan 6 '17 at 9:00
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I toured Shikoku recently. I was also nervous about the tunnels, but had no problems at all. Japanese drivers are very safe and polite, and on Shikoku seem very cautious in tunnels due to the many pilgrims walking the island's 88 temples: http://i.imgur.com/4fsIG1a.jpg I don't recall the air being particularly bad, either.

Most tunnels also have sidewalks. I ran into one 1km long bike-pedestrian tunnel that I had all to myself in the complete middle of nowhere— Japan loves to overinvest in infrastructure: http://i.imgur.com/6Yls6CP.jpg

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