Outdoor Lighting and Crime - Amper

ank places. Of course, if all cities reduced their waste light by the same amount, no change
in rank could be expected as a consequence, but overall crime could be expected to reduce, a
beneficial effect.
Although San Diego is not one of the 21 cities in the present analysis, the same method can be
used to predict the effect of the relighting discussed in Section 5.1 above. San Diego had a
1998 Morgan Quitno crime score of 7.46. Obviously there is a problem with the linear
regression line in Figure 10 as it would need to be constrained to a steeper slope in order to
avoid negative light losses, or a logarithmic light scale could be used instead. Ignoring these
problems for the moment, the regression equation for Figure 10 indicates that for each 10%
increase in ambient light and all else remaining unchanged, San Diego’s Morgan Quitno
crime ranking would increase by three places or more, again hardly the outcome intended by
the city authorities and the brighter-lighting lobbyists.
The light increase that will result from the already approved changes in street lighting may be
over 100%, leading to a predicted increase of well over 30 in the city’s Morgan Quitno
ranking, ie in the less safe direction. This predicted change is in the same direction as, but
smaller than, the change derived by a simpler method in Section 5.1. In either case, it would
seem to be a great deal of extra actual crime to put up with in order to feel safer than at
present!
Bear in mind that the UCR crime data and its derivatives are aggregates for day and night, and
include many offences not obviously subject to selective encouragement or discouragement
by light or dark conditions. The Morgan Quitno weighting process could possibly introduce a
bias for light or dark conditions, but no information is at hand to test this and it is hard to see
how any such effect could have much influence.
5.2.3.6 Is Tucson anomalous?
Tucson in Arizona has an innovative outdoor lighting control ordinance (eg Cook 2002),
which includes a limit on lumens per acre 62 (IDA IS91 1994). Although Tucson has a
population of over half a million, the Tucson-based International Dark-sky Association has
pointed out that the Milky Way can still be seen from downtown. This is consistent with the
widespread use of outdoor light fittings with limited or no upward direct light spill. One
effect of curtailed upward waste light would be a fainter city as seen from space, relative to
the amount of light on the ground. Thus, the Tucson data point in each of Figures 7, 9 and 10
could be expected to be to the left of the respective positions it would have had with more
wasteful luminaires, typical of those used elsewhere. Its data points in each case are indeed to
the left of, and above, the regression line, but not greatly so. Another way of describing the
Tucson data point positions is that they represent excess crime for the amount of light
measured by satellite. The distances of the data points from the regression lines are not
extremes within the data set. Therefore there is no reason to consider that Tucson’s lighting
or crime figures are anomalous. Changing the amount of light on the ground in Tucson can
be expected to have an effect on crime much like it would have in some other place. There is
plenty of scope for reduction.
62 One acre is about 0.4 hectare or 4000 m 2 .
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