Outdoor Lighting and Crime - Amper

night, plus the relative change over daylight hours. The transition between day and night
might need to be dealt with as some intermediate change over twilight hours. The net effect
would be an average of crime weighted by duration, taking account also of the absolute crime
rate represented by 100 on the vertical axis. With this information, eventually it should be
possible to construct graphs showing the relationship between the overall crime rate and the
mean light level at night in individual places of any size. Conversely, it may be possible to
work backwards from observational data of overall crime rate and light levels to derive the
general form of curve applying to the axis variables of Figure 6, bearing in mind the likely
presence of ‘smeared-out’ non-lighting social effects related to clock time.
4.2.2.5 Practical issues
So far, this discussion has related to light and crime at an individual place. But even with a
single streetlight and its surrounds as ‘the place’, the actual illuminance or luminance will
vary greatly with geometry in the vicinity. This raises the issue of what single light-related
value on the horizontal axis is to be used to represent the actual situation at night on a graph
of Curve E. The luminous flux emitted by the luminaire and related photometric quantities
such as peak illuminance take no account of the area lit or number of individuals served. The
use of quantities such as lumens per unit of ground area or light power per person would
appear to meet the immediate need for some sort of average value as a single number. The
place or area in consideration could equally well be under a streetlight, a town, city, groups of
cities or the whole of a country. The practical upper limit for size might be set not by the
light-related measure but instead by the homogeneity of the criminal justice system or
systems involved. Even then, some sort of continent, hemisphere or global figure might be
useful as a benchmark in research and planning.
The quasi-experimental work reviewed in Part 1 might now be seen to have previously
unrecognised faults, in that little or no attention was directed to features of Curve E that need
to be taken into account in the design, conduct and interpretation of such experiments. Future
experimental designs will need to incorporate sufficient sensitivity for reliable detection of
smaller changes in crime than those found to date. Treatments much larger than hitherto used
could assist, but run the risk of a confusing result if the actual effect varies rapidly with light
level or is non-monotonic. And, as emphasised in Part 1, much more effort needs to be
devoted to measuring the actual light levels at representative places in the experimental,
control and adjacent areas throughout the whole period of the experiment. In cases like
Painter’s Dudley and Stoke-on-Trent experiments, where the before and after periods were 1
year, for example, it will not be good enough to start the experiment at the before interview
period, just before the relighting treatment is applied. Monitoring of crime and light levels
must be in place for the whole of the before period and the whole of the after period,
preferably with the aim of plotting crime against actual light level as well as against light
phase, clock time or integrated time.
4.3 SPATIAL AND TEMPORAL RELATIONSHIPS
For all US cities with a population of more than 1 million, the rate of violent crime in the
central city areas is between 1.22 (Las Vegas, NV) and 7.63 (Milwaukee, WI) times that for
the respective suburbs, with an average of 3.22 (Demographia 1999). The Total Crime Index
rate for US rural areas is less than half that of metropolitan areas (Maguire and Pastore 2002,
Table 3.121). The situation is similar in other countries (Walker 2002).
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