Informal or natural surveillance is that which is done as a matter of course by any or all individuals within a space. Informal surveillance crosses public private boundaries. For example, people over see parks and streets from windows. Informal natural surveillance also occurs form public to private space e.g. where passersby notice activities in private spaces.
Studies of residential burglary demonstrate that burglars avoid targets that are readily overlooked by neighbours and/or passers-by. Properties with low levels of lighting at night, high walls/fences, or thick trees or shrubbery can provide concealment opportunities for burglars particularly when close to points of access such as windows and doors.
However, the existence of natural surveillance opportunities within the built environment does not necessarily mean that surveillance is routinely taking place, or that any direct action by citizens (e.g. challenging, reporting or direct intervention) is guaranteed. Indeed, this is one of the reasons for the development of second generation CPTED, which now seeks to engender positive social activities and diversity to encourage neighbours to take ownership of space and take advantage of natural surveillance.
Angel (1968) predicted that certain critical levels of street activity and population density were linked to crime. A critical crime “zone of intensity” was therefore one that could support low numbers of people but in sufficient densities to contain both victims and offenders. Loukaitou-Sideris (1999) has proposed that a second level population density exists; where the density is sufficiently high to mask a range of less serious offences such as pick pocketing and petty theft.