[5.4] Standard assets & nav meshes

Hi there,
I just downloaded Unity 5.4 and I wanted to improve the car AI scene to learn how to use nav meshes.
But nothing works : the nav mesh on my terrain is OK, nav mesh obstacles are carved but the nav mesh agent on the car still doesn't avoid collisions.
I suspect it is because of the AI controller but I'm new at unity and I really don't know what to do...
Can someone point me to the right direction please ?

Thank you !

 

In fact I'm trying to achieve what this guy made on this video :

What I tried so far :
- create boxes, assigned a Nav Mesh Obstacle to them
- put the Nav Mesh on the car, but also in an empty object as advised in a comment of the above mentioned video
- put the Nav Mesh on the WaypointTargetObject, as it is the object responsible for the trajectory
- play with the settings

Any help or advice would be greatly appreciated as I'm new to Unity