for Transport Studies
to correspondence between Simon Tonks and Susan Beck I have been asked
to write to you and I have been assured that you will be able to answer
questions in some detail about the government document: “A cost recovery
system for speed and red-light cameras ~ two year pilot evaluation”.
major concerns that the document is highly misleading about the benefits
of speed cameras and I would appreciate specific and detailed answers to
are there no fatality figures? I quite understand that the numbers may
be too small in some cases for statistical significance, but county-wide,
and pilot-area-wide figures would have been useful and would have revealed
some serious anomalies.
to the mean
did you feel able to claim 35% reduction in KSI at camera sites, when you
very well know that most of that “reduction” is a “regression to the mean”
benefit illusion? It is obvious that the selected method for choosing camera
sites will specifically encourage regression to the mean errors. No compensation
for site level regression to the mean has been applied, and as far as I
can tell, no investigation has been carried out even at sample sites to
determine if the effect was significant. I am drawn to the following text
in the report:
could not obtain data for the before period for individual sites other
than at camera sites. It was therefore not possible to check fully for
regression to the mean at the site level. The results for areas that bid
unsuccessfully for participation in the pilot could be used as a comparison
for what might have occurred in participating areas if they had not been
treated. The PIA and KSI frequencies for these areas do not differ significantly
from other similar areas that did not bid for pilot status at all. On this
basis, there is no evidence in the present data for any substantial illusory
benefit due to the regression to the mean effect.
are weasel words. I expect having insufficient data to check for regression
to the mean benefit illusions was a considerable relief to those who wished
the report to show a benefit.
for traffic reductions at camera sites
much of the claimed headline benefit is due to traffic reduction? I can
find no mention in the report of reduced traffic volumes at camera sites.
Since it is well know that drivers prefer to drive on routes where there
are no cameras I find this quite astonishing. In cases where suitable alternative
routes are available installation of cameras can lead to quite substantial
reductions in traffic. Naturally with reduced traffic on a given route
we would expect there to be fewer accidents.
area selection trick
if you noticed that the pilot areas taken as a whole showed untypically
bad performance in 1999, compared to the rest of the UK? (Especially for
fatal accidents.) This has tended to incorporate a wide-area regression
to the mean benefit illusion that has not be compensated for.
are the ranges of sizes of the “camera sites” considered? Are the cameras
really responsible for reductions in accidents at the most distant extremes
of the sites? Compensation for traffic volume reduction is important here.
much of the benefit claimed for cameras is due to other safety treatments?
It is very common for a speed camera to be added to a road at the same
time as other safety treatments. No mention is made in your report of other
safety treatments applied at the same time as the cameras. This must be
causing an error in your headline figures of unknown size. It could be
very substantial. It is entirely unreasonable to assign the entire benefit
to the cameras, but that is exactly what you have done.
doesn't the report address casualty displacement properly? It is obvious
that speed cameras cause casualty displacement. A number of mechanisms
are well known. In particular:
falls very far short of adequately considering the extent and the implications
of casualty displacement.
divert to alternative routes
attempt to make up time away from the cameras
attitudes may be worsened by cameras
priorities are distorted by cameras
you look at the trial counties over say 15 years and taken as a whole,
which years show the greatest improvements? Were they the years of the
hypothecation pilot? (I can tell you they were not.) This alone is a very
strong warning that “things other than cameras” are better for road safety.
speeds at camera sites
do you believe to be the benefit of reduced speeds at camera sites? The
report makes much of the reduction in speeds at speed camera sites. A linkage
is made to TRL421 and the claim of a 5% reduction in accidents for each
1 mph reduction in speed is repeated. But I expect you know TRL421 is not
justified in making its claim. No evidence is offered that the reported
speed accident relationship has a causal link. Do you have any substantial
evidence that a reduction in speeds due to enforcement causes a reduction
in accidents? References please!
there are negative economic effects of reduced speeds. Time, after all,
is money. It is a great shame that these economic effects were not considered.
have some further questions depending on how we got on with these. I look
forward to hearing from you in due course.