Quite some time ago, I suggested scouting your area if you live in an suburban or urban environment to look for gang activity. My scouting has involved nothing formal, just taking different ways to the grocery store, driving behind buildings to scout if they have been tagged, etc. Nothing too crazy. I have a large police force in my town, and the community is pretty good at not putting up with groups of youths running around causing mayhem.
On an abandoned building I saw this:
Using the gang identification link I learned this is none other than MS-13 in my area. That's just great. I have not seen any other gang activity, which makes them more dangerous. Somewhere in town there are a few houses loaded with guns, drugs, and nitwits who are just waiting for law and order to break down.
If you live in a suburban or urban environment, knowledge of what area is claimed by who will have a direct impact on your survival in a lawless situation. I will be assembling a map in my survival book with notations on where and when I have seen gang graffiti and other activity.