London Cameras- No connection between cameras and clear up rate

"The principal measure of effectiveness as far as the Police Service is concerned is in relation to the support of the investigative process. [...] Interestingly, there is very little academic research on the effectiveness and usefulness of CCTV in the investigation of crime, most of it is focused on does it reduce crime, not what is the impact of it in terms of investigating crime."
The statistics show that more CCTV cameras don't lead to a better crime clear-up rate. This is best illustrated when the figures are plotted as a graph (see below) which shows that there is no linear dependence between the number of cameras and percentage of clear-up.
London has over 10,000 state run cameras.
Graph of number of cameras vs. percentage crime clear up rate
(Prepared by No CCTV using the figures obtained by the Greater London Assembly)
http://www.no-cctv.org.uk/caseagainst/london_cameras_versus_clearup.asp
http://www.youtube.com/watch?v=fNscBk0n9lc