Map

The map allows you to travel between locations. In the top right corner, note the option to select method of travel: Foot, Horse, Car. The two latter options you have to acquire before using them and both increase your speed and therefore your options of where to scavenge. In the bottom right corner, you will see what time it is. Make sure the main character is back at your shelter before 11PM to avoid Fatigue (NPC party members get fatigued at 8PM).

Accessing the map
You can access the map by moving a character to the Blue Grid of a location and pressing "M". When leaving your shelter, your character must be in the party to leave.

Description
Characters with higher levels of the Survival skill will be able to spot wild sources of food on the area map. If you know where they are, you can travel to them, harvest the food (automatic, not pick everything up manually), and come back if they pop up again.

Random encounters may pop up and be marked as you travel, but won't stay on the map if you ignore them. All locations you have gotten added to the map will tell you how many times and when you have visited them before.

The group's foot and horse travel speed is based on the sum total survival skill of the scavenging party. Car travel speed is not affected by survival skill. (tested in game: On foot: Splendid to South Truck Stop-Total skill 8 in group of 3 or 4 was same at 3h39min. Average skill 3 was different: 3 people/9 total skill was 3h23min, 4 people/12 total skill was 2h48min. By Horse: 3 people/9 total skill was 1hr35min, 4 people/12 total skill was 1h19min. By Car: 4 people with 9 total skill or 33 total skill was the same travel time).

Horse travel with total group survival skill of 29 (excluding Priscilla) is equal to the car for travel speed. So total survival skill of 30+ on horse is faster than the car, making the car only useful for carrying an extra 200lbs of items at the cost of fuel later in the game.

Locations and Loot
The loot found at locations to some degree match the type of location. Supermarkets have more food, hospitals more medicine, etc… Loot is finite. The only reason to go back to a location is if you didn't get everything the first time.

It’s possible that a location could have fewer resources over time or the small chance that something else could have happened to clean out the area entirely. There are always urgent reasons to do almost everything in the game - that's part of the decision making process.