Imagine/Safety
Looking at the Map of Malton Suburbs, as well as the game on a whole, its clear the "safety" is a major concern to survivors.
But what does "safety" mean in Urban Dead? The above map seems to define it in terms of the number of zombies around, with an eye to determining your chances of being killed on a given day (although it makes zero mention of PKer activity). A similar analysis / reasoning is given by the stats page, with the Survivor / Zombie ratio often being taken as a sign of how "well" survivors are doing.
I think a different metric is needed. Safety is the freedom to live. While it might seem that getting killed would prevent this, it in fact does not; If you have "ankle grab" it only costs you 1 AP to stand up as a zombie after being killed as a survivor, and then another 1 to stand up after being revived. So IMO, "safety" should be measured in terms of how long it takes to get revived when you desire it; the longer you are forced to spend time as a zombie against your wishes, the less safe you are as a survivor. Some Red parts of the map have (in the past at least) been very safe in this regard, while some green zones are in fact not very "safe" in these terms.