Problem with lightmapping artefacts

Hello all !
First of all thanks for opening this complete noob post,
I guess this is the correct place to post this help request, if I am wrong please let me know and I will move the thread.
So here am I, starting my first unity project and for now what I have is just a rough idea that I want to continue developping.

I'm making a sort of architectural interactive scene that takes place on a highway, in the Netherlands.
So I made this quick model in Blender, of the scene and a few objects that dwell on it (which may not be optimized at all, so that could be a solution to the problem).
But here is the problem, I built and ran the game without any pre-baked lightmaps and the rendering was "ok" for a demo. But then I wanted to know what the game would look like when baked, and thus I started baking lightmaps with progressive lightmapper.
Although, something weird happens, as you can see on the image I attached. The whole inside of the highway portion (the inside of a roundabout), is completely darkened. As if there was no light nor shadows or material.

I reduced the settings of the progressive lightmapper in the lighting panel, just to get a quicker bake, but could that be the reason why it is "weird" ? Or is it just because the model is badly modelised ?

Please let me know, and thanks again for taking the time to help.
Lucas


EDIT : Hi guys I'm definitely not in the good section haha, how do I change that ? Thanks