Navigation Volume

I have a map of a city made in Unreal. The roads and sidewalks are one static mesh however they use different materials. I am trying to get people to walk on only the sidewalks. When placing a navigation boundary across the whole map, obviously, the people are able to walk on the roads. The map is too large and too complex to go through and place nav modifiers everywhere to negate the boundary. How can I get only the sidewalks to be “walkable”.