It seems a bit unrealistic to me that the metro tunnels, and other areas, remain completely devoid of life once I clear them out. While I don't want them to respawn completely, I figure there should at least be some Radroaches running around when I go back to the area later. What I'm wondering is if an encounter zone I give to a Radroach leveled list would override the zone placed on the area itself, or would the area zone take precedence?