Ignore the word tunnel, i meant just a passage underneath.
Classic tunnel is easy to spot when it long. Sort of the “light in the end of the tunnel”.
Here is few photos its not so clear if the structure is a tunnel or is it a bridge above:
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I’m not sure if there’s specific tag for such passages, but I guess tunnel would make more sense in this case, as it’s below earth - type of ground.
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In all of these cases the road is for me the ground level. All the animal crossings are bridges, except for nr. 8, where it is a tunnel.
The road does not elevate and the structures look like they are build over it. This means for me, they are bridges. This might be different for tunnels through mountains, where there is no elevation either. But I think it always depends a bit on a personal view.
From a constructions companies pov these are all bridges except of no. 8 (agreed with @Robert46798). We had a similar topic recently in the german forum and there was some kind of a consensus to tag the main structure as man_made=bridge + layer=1 and on top man_made=wildlife_crossing + layer=2 + natural=srub/grass or whatever is applicable. Additionally fences, noise barrier walls and tracks (which sometimes exist implemented in the wildlife crossing) as far as applicable. Example:
Way: Grünbrücke Teupitz/Tornow (242850373) | OpenStreetMap
One may argue in case 2 and 7 the street is located in a cutting - this is not ground level.
Yeah, on a second look I realized this too. But from the appearance this is a bridge over a street. - And now the differentiation becomes very vague, what was the origin of the post 
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I remember an example of a tunnel on a bridge, just one road (crossing water) 
yes, and the lower road as cutting.
A tunnel is dug into the ground (always tunnel), or sometimes it is constructed openly and covered afterwards, there are (different AFAIK) criteria to determine whether these count as tunnels in specific jurisdictions, you would have to ask a civil engineer about it, and there will be precise definitions.
For osm, I do it like this: if there is a bridge structure above, it is a bridge and not a tunnel, if it is dug it is a tunnel, if it is closed after construction but quite long it is also a tunnel
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yes, for me it looks like bridge - and Suomi24 Viihde - Uutiset seems quite clear case of tunnels (maybe except one with elephants)
Why it would be relevant? What about round going in tunnel though 30m high embankment - would you also mark it as a bridge because road is at ground level?
Is it really supposed to be a bridge: https://www.railadvent.co.uk/2022/06/watch-timelapse-footage-shows-a-railway-subway-being-built-in-sussex.html ?
It is at ground level.
In my opinion it is a tunnel.
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It was just my first approach to differentiate those two. To get a better understanding I looked to Wikipedia:
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end.
So look for the ground level to whether something is underground sound reasonable to me
On the other hand you provided a fine example with the embankment, where the way goes through soil, although it is on ground level. As it is also mentioned as tunnel in the article it seems feasible to see it as tunnel.
tunnel=building_passage is also an example where the ground-level-approach does not work.
Therefore: If the way goes through soil, earth, rock it is likely to be a tunnel.
Remaining with the provided Examples: Especially picture 5 is hard to categorize for me. Still it looks more as if the structure is build over the street, so therefore it would be a bridge for me. But this may influenced by my language/experience, as in Germany this would be called a wildlife bridge 
Considering the said:
I would ask for a specific way whether it was build to get through a structure (ground, building, embarkment, …) =tunnel, or to get over it =bridge. In this case the length or transport mode would not be relevant.
The embankment case make me the most uneasy with classification. My gut instinct is to ask “does it look like it’s supporting soil which itself supports other things or does it look like it support the road/footway directly?”. If the former I would tend to say tunnel even through an embankment (with some nervousness), the latter a bridge even if in over a cutting.
Even this approach has its difficulties, many traditional stone bridges look a bit like tunnels through fill even though they are definitely bridges. I suppose they get too thin at the top but with barriers at the sides that’s not easy to see at “river level”.
The wildlife bridges are an odd case as they are clearly thought of as bridges even though some look like tunnels through embankments to me.
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Funfact: in the GDF (a navigation data format) they use the class brunnel (Bridge or Tunnel) to avoid these discussions.
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For tall truck, or large transport, it matters a lot. Unless you want to break your vehicle. 
We map that separately as maxheight anyway.
There’s already been quite a bit of discussion, and it’s becoming hard to follow for someone like me who needs to translate.
However, I’ll try to write down what I consider to be the criteria for a “bridge” and a “tunnel”.
- If the structure below forms the floor of the structure above, it is most likely a ‘bridge’. If they’re separated, or there’s something like dirt between them, it’s a tunnel.
- If there is a clear direction and there are railings, it’s likely a bridge; if there is little direction and there are no railings(there may be a fall-proof bump), it’s likely a tunnel (E.g.: covered river).
I usually to determine whether I am going through or over an existing space. If you need to cut in it, like a mountain than it is tunnel. The most common example is a roadway with culvert runing underneath. The culvert is a tunnel because it cuts though the existing road surface.
If you pass over a empty space, then use a brodge. Things like railing are usually associated bridges. They can also be used elsewhere there is a need to keep pedestrians away from nearby dangers. If one of those are falling then the structure is probably a bridge.
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The culvert is a tunnel because it cuts though the existing road surface.
I always thought tunnel=culvert was kind of a hack for carto in pre-hstore times, not that culverts were actually tunnels. If water flows through a tunnel, there will usually be some air above (free flow).
I think your right when it comes to underground waterways. Id seem to want them in tunnels even if they is no reason to believe that is the case.