Lighting effect

Question

The following screenshot is taken from a road that consists of several equal parts:

They all belong to a Blender model and have Shading set to Flat.
The scene has just one directional light and the parts of the road are used to bake a light map.
Material is set to use Mobile / Vertex Lit (Only Directional Lights) shader but I tested other shaders like Mobile / Unlit as well.
I don't get this effect when I take a model consisting of 8 road parts using a repeating texture - at least not in the middle. So it's not related to broken tileability of the texture. But it is visible then at the end as well The texture import settings are:

Question: Is this the regular behaviour of the standard shaders? Can't believe it, but If so how to omit this when writing my own shader?

I would appreciate if someone could shed some light on this ;-)

Answer 1

It's a problem with normals. Easy fix is to make Unity recalculate all the normals (In model import settings switch Normals -> Import to Normals -> Calculate).

Answer 2

hard to know without seeing more context, but if the objects are lightmapped, make sure to check that they are not reusing the same lightmap texture. @jaceck's suggestion about normals is a good one too.

Answer 3

I do not know if it is this you want, but there is also a component called Line Render that draws a line.