NavMesh doesn't cover intersections

I have imported the roads (it was a single FBX file, all the roads are seen as one object) and despite playing around with the bake parameters, there is always a clar, albeit small, diving surface between intersecting paths which basically locks any AI to walk on a single road and get stuck as soon as they hit a cross section.

Any ideas on how to fix this?

 

The built-in navigation system leaves a lot to be desired. I use NavMesh Components, which I gather is written by Unity Technologies, but not included in the editor by default. It gives you a lot more control over baking and also allows you to add a nav mesh to a moveable object - and even save the navmesh as part of a prefab. You can find the documentation here. Be aware that Unity considers it experimental, hence why it's not included in the editor, but I've had a lot of good luck with it.