Salut Marc,

How should we tag roads like motorway A5 with N5 above ? http://maps.google.ch/maps?f=q&source=s_q&hl=fr&geocode=&q=n...

Is there a tag for this case?

There are two problems in your case:

1) How to tag roads that overlap for a certain distance:

As André wrote (and as it is at the moment in OSM), the layer-tag can be used to assign different layers. Personally, I would not sketch the lines that close but leave a gap of about 5 meters. Right now, the motorway disappears completely under the primary road on the rendered map..

2) How to tag a "half-opened tunnel"

AFAIK there is no tag yet for this. For rendering-purposes, I would keep the tunnel=yes and add something like

gallery=yes|left (*)

or

open_sided=yes|left

and document it in the Wiki.

Regards, Thomas

(*) Brings the problem of ambiguity I'm not even sure, if 'gallery' is understood by an English native (it's 'Galerie' in German).