When is a bridge not an overbridge?

See this definition:

"Where a bridge takes one form of transport over another it is both an overbridge and an underbridge, depending on the reference level. For example, where a road passes above a railway, the bridge is an overbridge from the point of view of the railway and an underbridge from the point of view of the road."

http://en.wiktionary.org/wiki/overbridge

I think this definition is confusing. It should be the other way around. From the point of view of the railway, it should be called "underbridge" because the bridge structure allows the train to go under the road. And, from the point of view of the road, the same structure allows the cars to go over the railway. In other others, it should describe what it allows you to do as you use the structure. The other way is unnatural, because you are thinking from the point of view of the other, what the structure allows the other party to do (go over/under me).

The terms "overpass" and "underpass" are used in the way I describe. It's an overpass if it allows YOU to go OVER something. It's an underpass if it allows YOU to go UNDER something.

So, it should be called "overbridge" if it allows YOU to go OVER something, and "underbridge" if it allows YOU to go UNDER something.

These terms seem to be British, but I (a Brit) hadn't heard them before, and I think they're probably more technical terms.They're both in Oxford Online:

overbridge - a bridge over a railway/railroad or road
underbridge - a bridge spanning an opening under a railway or road.

This suggests to me that an overbridge takes the road or whatever to a higher level to cross something at ground level, whereas an underbridge takes the road at the same level over something passing at below the existing ground level.

At first this example sentence form Oxford seems a bit confusing - "The overbridge was built to replace an underbridge which restricted lorry access" - But I think what it means is that previously a road went under the road in question, which stayed at the same level it was on either side. But as this didn't give enough headroom for lorries, a new bridge was built, which raised the level of the road in question to a higher level - hence an underbridge.

On the other hand here are the definitions from a glossary of civil engineering terms, which suggests that the terms over and under are relative to a railway.:

overbridge - a bridge that spans a railway - ie. something goes over a railway
underbridge - A bridge that supports a railway track over a road, river or other obstruction - ie something goes under a railway

http://www.rgsonline.co.uk/Railway_Group_Standards/Infrastructure/Codes%20of%20Practice/GCRC5603%20Iss%201.pdf

My point is not about the accuracy of either term, just that neither is is in common use in the majority of the English speaking world.
It does seem that in NZ the civil engineers are quite fond of such words.
They do for example seem to favour the word deviation over bypass or detour when describing a road going around rather than through a town or village.
A phrase like "The Hobsonville Deviation" conjures up images other than a stretch of asphalt.

@HS - as to common use, you maybe answered your own question there, as you say in NZ "civil engineers", (from which I assume not necessarily the general public) are fond of the term.

The document I quoted definitions of overbridge and underbridge from above is called British Standard Definitions of Civil Engineering Terms, and is published by Railtrack, the company that owns and runs the railway infrastructure in the UK. They say the document "defines terms in common use in civil engineering". So I think we have to assume that civil engineers in Britain are "quite fond" of them as well.

I think "deviation" is a separate issue, however. This does indeed seem to be another of your NZ curiosities - "The Hobsonville Deviation" gets more than 20,000 hits on Google, as opposed to 49 for "The Hobsonville bypass". Oxford Online doesn't include this definition of deviation; the Onlinedictionary.co.nz, however does include this: "a diversion from the main highway", which I would call a detour.

On Google, "deviation motorway" brings up mainly hits for Hobsonville, but also a few for Greenhithe, Mangatawhiri and Maramarua, all of which seem to be bypasses on NZ state highways. Interestingly, NZTA, who run the state highways and refer to the Maramarua Deviation, on the SH2, are planning something similar on the same road to avoid Katikati, but they refer to this as the Katikati Bypass. Is there a difference? And is 'deviation' only really being used as a technical term during construction? - NZTA's own map simply refers to the Hobsonville Motorway: http://www.nzta.govt.nz/projects/hobsonville/interactive-map.html

There are also plenty of NZ hits for "deviation road". And incidentally, there's a winery in Australia called Deviation Road Winery.

Just like my mistake: 'everybody have' instead of 'has'. . . for example.

I think that, from a rigurous logic/semantic point of view, it keeps being a very ambiguous or slippery distinction, and we'll keep getting lost in the ambigüety of the use of concepts or, rather, in the words we use to express these.
the description 'a bridge that passes underneath a railway' can bring to one's mind more than just one picture. 'a bridge that supports a railway line (or railroad track) while crossing over a road or a river or something. . .' would probably evoke fewer pictures, or maybe just one with a bit of luck. If a word or sentence can evoke different pictures in your mind, or in different people's minds, well then it's not very rigurous from the logic/semantic point of view, but the English vocabulary -as well as every other language's vocabulary- has lots of terms that can be judged to (not) be so.
So, we could just conclude that everyone above has been right in their opinions from their own respective standpoints, and that, of course, it is very necessary to establish a fixed glossary of terms (no matter how semantically unsteady) to mean exact different things so to prevent unsafety (for example when handling trains or other vehicles, or machines, etc.) or confusion. So, be it as it may, this is a human society -that's to say, a very imperfect thing- but we just have to do what we can. . .

the 2 last posts have been uploaded in the inverse order: the 1st was the 2nd, so I just corrected that mistake. . .

also 'ambiguity' is the correct word. Sorry, my english is VERY rusty by now.