A Safe Zone is an area within Dying Light where the infected cannot (usually) gain physical access to.

Overview

A Safe Zone as the name implies is an area where survivors can do things without worry of any infected being a danger to them. They are protected by UV Lighting to prevent Volatiles/Night-infected from entering or getting too close and often have other defensive measures such as barbed wire/barbed wire fencing, electricity traps, or are positioned in otherwise physically inaccessible locations for the other infected. A noticeable example of a safe zone that relies mainly on UV light to keep the infected out/from entering is the Tower. All safe zones rely on UV light around their perimeter/main entrance i.e. the safe zone where the incomplete highway going into a raised tunnel in the Slums has UV light on the exterior/facing out and away from the tunnel, pointing out to the only point of access and subsequently keeping out the infected (particularly Volatiles). 

Safe Zones are represented by a green house symbol on the game map. Most feature beds which allow the player to skip to morning/evening, a player stash bag to access a larger inventory, and a bounty board to access challenges. Many larger safe zones also have stores, side quests, and in some cases a quartermaster e.g. Harran University (Spike) as well as the Tower (simply identified as "Quartermaster").

Safe Zones that have not been unlocked yet appear on the map as a red house symbol, and often require the player to kill several Biters, block an infected/friendly entrance such as a doorway or opening in the wall, and activate the power via a fuse box or breaker switch.

The Safe Zone will become safe only when all the requirements have been met.

Some Safe Zones will only be unlocked as part of side quests (such as David's flat in Old Town during the side quest "Legless Spider").

Some other Safe Zones will become permanently unavailable beyond certain parts of quests/upon quests being completed i.e. the safe zone just south of the Tower (with Zere's research truck within the perimeter) will become unavailable once Rais' gang members attack it and destroy its defenses/barriers. From this point onward in the game, those locations can no longer be used as Safe Zones. 

Safe Zones such as the Embers' Towers/Safe Zone in Old Town have soft landing spots/grouped trash-bags purposely placed below them for when runners/scouts need to quickly jump down from the safe zone.

General Features of a Safe Zone

1. Protects the player and any other friendlies inside from outside dangers. 
2. UV Lighting Around the Perimeter/Entrances (Prevents Volatiles Entering)
3. Allows the player to skip through the hours from either night to morning or from any time during the day to night via a sleeping bag/bed.
4. Being able to deliver/give airdrop supplies to the safe zone's quartermaster. 
5. Restock on supplies from the safe zone's shop/trader (if there is one present). 
6. Interact with NPCs within the safe zone. 
7. Sell scavenged items/resources/weapons/supplies to the safe zone's shop/trader. 
8. Used for Evading Pursuits (at night when chased by Volatiles/Virals). 
9. Storing equipment, items and weapons in the player's stash-bag. 
10. Check on the Bounty Board (doesn't apply to all Safe Zones). 
11. Check on challenges (most challenges are only available during the day).

Requirements for Clearing an "Inactive" Safe Zone 

Note: When coming within proximity of an inactive safe zone, the game will inform the player(s) that they are near an inactive safe zone and will prompt them to try and clear it. 

• Clear Area of Infected - Often several Biters and sometimes a various types of infected.
  • Mostly, the variable types of infected are probably one or two Virals, maybe one or several Gas Tankers and in some cases such as one of the playground-areas in the Slums, one Goon.
• Close Entrance(s)/Possible Entry Points for Infected - This can be a massive door to a yard area (like the aforementioned playground-area in the Slums to the North-East of that sector) or a bookcase being shoved infront of a doorway/window. 
• Activating the Power (for the Zone's UV Lighting) - This will either be a fusebox (where Crane will screw some fuses into place) or a circuit breaker (where Crane will flip all switches in the breaker back on before activating the main power switch). This activates the UV lighting for the safe zone. 

Upon meeting these requirements, the safe zone is unlocked. 

Exceptions

Sometimes, safe zones will only be unlocked after a certain part of the story or quest objective (both base game and in The Following) i.e. upon clearing out one of Rais' outposts in Old Town, the outposts are then converted into safe zones.

Likewise in The Countryside, most safe zones will only be unlockable/usable after the quest Power to the People (as this quest is where power gets restored back to most if not all locations around the Countryside). The only locations that can really be used as safe zones until the quest "Power to the People" is completed are Jasir's farm and Bilal's gas station. 

Hints/Other Information

• Safe Zones allow you to evade pursuits straightaway due to the Volatiles/night infected being unable to chase you into the Safe Zones.
• In order to minimize use of melee weapons, firearms and throwable equipment/throwable weapons, for some safe zones you may want to just kick the infected off a ledge (such as a rooftop) or dropkick them over the edge of a rooftop. It also works very fast when done correctly. 
• Kyle will comment on some beds, saying it looks comfortable.
• In some parts of an unlocked Safe Zone, you can still use weapons/equipment to attack infected that are either below you on the outside of the perimeter or are in an area where you can shoot/use ballistic weapons against them.
  • In this respect, Safe Zones can be used as a means of "cheesing" for Power XP (where there is minimal effort or difficulty in levelling up the player's Power XP). 
• Volatiles will sometimes do an ironic thing of walking/jogging into a safe zone, burning up and glowing orange due to the UV light being shone onto them. 
  • This is likely to happen in places such as the Tower with Volatiles clumsily entering the Tower's ground floor only to become frightened and in pain at being burned by UV light. 
  • Getting in the way of a Volatile fleeing UV light (even though you can be in the safe zone) can still lead to the Volatile hitting you - Not pouncing you however, as the Volatile will want to get out of the UV light before anything else. Only when you leave the Safe Zone (with the Volatile seeing you and chasing you) will the Volatile try to pounce-attack you. 

• In The Following, you cannot park your buggy in the Countryside's safe zones - Whilst it is simple to just park the buggy and leave it very close to the safe zone, it cannot and will not physically get into the boundaries of the safe zone's "safety area" (where you cannot use weapons or equipment bar the UV Flashlight).