Create nav mesh on top of static mesh actor

I am trying to play with a car that drives itself around a city with path planning.

I have the city built and the road network is a static mesh actor. I can create a nav mesh, but I don’t want it to be on the entire level… The car shouldn’t drive on grass, sidewalks,etc. I really only want it where the road object is.

Is this possible? Or is there a different / better way to do it?