So what you need to do here is look at your texture without the UV map on. Ensure that it tiles flawlessly, if it does not this is source of your problem.
To solve this you want to make sure that A will always line up with B. I suggest doing this by essentially stitching together the joint on B where these images meet, then when you are sure that they tile, discard A and use B as that is the fixed tile.
I'm quite sure that this is your problem as even in the Unity preview you can see that there is a blemish between the two yellow lines that goes in a straight line right up as far as the kerb,if not as far as the grass, the yellow line on the left is also lower than the one on the right.
In the blender preview you can see hints of this blemish on the left and the tile doesn't really tile as a result.
Tiling is a skill in and of itself, just using the seamless filter in Gimp is not enough, though it might improve some seaming issues, you will still need to tidy others.
I would suggest utilising the clone tool and perhaps the smudge tool to help improve your results. I would also suggest double checking that your tile tiles by lining them up next to each other as mentioned above before popping them into either Blender or Unity.
If you do not have the time in the context of your project to facilitate for tiling you could work around it by using a gray fill for the road and adding random speckles, texture,dirt etc. rather than having the texture as a consistent pattern.
Hope this helps your project :)