Evacuation Route?

Several times each week I have to go up and down a portion on I-83 to deposit and collect my children. Upon returning (heading northbound away from the city) I always see this small white sign with a blue circle with the words "Evacuation Route" and an arrow pointing heavenward. What does this sign mean? Who is it for? Evacuate from here-to where? Are there more of these signs on other roads? Is this one of the security measures instituted post 911?

If you have any ideas, I want to hear them. Post, post, post. :)