A 10-km-long route.

I would say "10km-long route".

http://books.google.de/books?id=8n-GDx3cGHMC...
Hard to find much on that specific question; this is all I can find. Do you have any sources, -JP?

SveRü

Nothing to do with grammar, but with typological convention.

However, personally, as the route is clearly not 10 km high, or deep, or wide, I would just say 'a 10-kilometre route' or 'a 10km route'.

Definitely with two hyphens if you use the word 'long,' not just one, because the whole thing forms a compound adjective. While you sometimes see units written without a space (e.g., 10km), I don't believe most style manuals recommend it.

However, it would help to have more context. For example, if it were for running, it would be a 10k race, so there would be no need to mention the length of the route separately; you would use other descriptive adjectives, like a hilly route, urban route, etc. If it were a permanent location for practicing racing, like for mountain bikes or cross-country running, it might be something like a 10k trail or path.

If you did need to use the word 'long,' then you should normally write out the word 'kilometer,' e.g., After you go over the pass, there's a ten-kilometer-long stretch of very curvy road, with one switchback after another.

I can't think offhand of any reason you would need to write 'ten-kilometer-long route,' but perhaps if you shared your context, we could check and be sure.

Thanks to all for the comments, in particular to hm for his detailed explanations and to CM2DD for linking to a very interesting book.

The full sentence / context is as follows: "The transmission line is a 580-km-long subsea cable which makes this system the one with the longest submarine power cable in the world." After having read the linked style manual and hm's comment, I'm quite sure that this syntax is correct.

(However, in running text "580-kilometer-long subsea cable" would have been a little bit more appropriate.)

Und warum brauchst Du da das "long" überhaupt? Ich würde es einfach weglassen.

agree, "long" is redundant

I'm not actually so sure that 'long' is redundant. For some reason it sounds better to me with it in.

#9 Obwohl in der zweiten Satzhälfte auch nochmal "longest" kommt?

Yeah, pretty much. Remember that we weren't all trained to avoid repeating a single word.

For some reason it sounds better to me with it in.

to me too, as it happens, but I would still delete it to make the text as concise as possible. And the pedant in me is nudging me too :-)