Oak Ridge:Please Hire a Qualified Accident Statistician!

March 1, 2010

Letter to the Editor

by Dale Gedcke, B.Eng., M.Sc., Ph.D.

The following open letter was sent to the Oak Ridge City Council

The 1/23/10 news report by Beverly Majors in The Oak Ridger (Crashes drop, cameras cited) quotes excerpts from the 1/21/10 Memo to the City Manager, No. 10-01 regarding December 2009 Traffic Camera Statistics.  There are a lot of details that are worth analyzing in that report, and in the previous reports for October and November.  A copy can be obtained from the City of Oak Ridge.

For simplicity, this letter addresses only two issues.  First, the report implies that the installation of Red Light and Speeding Cameras in June 2009 is responsible for reducing the number of accidents in ALL of Oak Ridge by 13% compared to 2008.  That conclusion is fallacious, as can be easily demonstrated by combining the accident numbers from the December report with numbers previously published by the City of Oak Ridge on 6/2/08.  In 2005, there were 1,945 reported accidents in Oak Ridge. In 2006 that number decreased by 3.6% to 1875 accidents.  That rate of decrease accelerated to 22.7% for 2007 with 1,450 accidents.  Even more dramatically, the accidents decreased by 30.3% to 1.011 in 2008.  Then, the rate of decrease shrank to only 12.4% in 2009, with 886 accidents.

Why was the rate of decline in accidents accelerating from 2005 through 2008, but shrinking in 2009?  The cameras were not installed until 2009.  Did the cameras put a stop to the decline in accidents in 2009?  Why was the decline only 12.4% in 2009, instead of the 30% decrease established in 2008?

http://www.oakridger.com/archive/x1775162829/g113000de5f81da12397f30cebb9737333e04d1d503820f.jpg

Ironically, there is no causative proof that the cameras were responsible for restraining accident reductions in 2009.  Correlation does not prove causation.  For that same reason, it is fallacious for the December 2009 Camera Statistics Report to imply that the cameras were solely responsible for the 12.4% decline in total Oak Ridge accidents in 2009.   

The intersections and school zones with cameras are a negligible fraction of the intersections and roadways in Oak Ridge.  Furthermore, via roadside signs and publicity, most drivers in Oak Ridge know where the cameras are, and drive through those specific areas with extra camera caution.  Outside those regions, they relax to their normal, safe-driving habits.  Consequently, it is not rational to assume that the cameras have a significant effect on accidents in ALL parts of Oak Ridge.  Rather, one should compare the accident rates per million vehicles through the intersections having cameras to those same trends for non-camera intersections of a similar nature.  That is what all the sound statistical traffic studies do.

Secondly, the number of accidents at camera intersections is so low that it is virtually impossible to have a statistically significant comparison from year to year.  For example, there were 24 accidents for the last 7 months of 2008 at the Oak Ridge Turnpike and LaFayette Drive.  For the same period in 2009 there were 16 accidents with cameras installed.  After removing the 12.4% reduction experienced for all of Oak Ridge, the remaining difference in accidents is 5.  That is well within the uncertainty expected for random variations in accident rates.  (For those steeped in statistics, the difference is 0.83 sigma, a factor of 2.4 less than the required 95% confidence level).  Consequently, one cannot conclude there is a statistically significant reduction of the accident rate as a result of the cameras.

For more details on this analysis, see the article, The Placebo Effect and RLCs, at http://tnliberty.org/wordpress/?page_id=40.

To avoid misrepresentation and embarrassment on published accident statistics in the future, the City of Oak Ridge should hire a certified traffic consultant who has expertise in statistical analysis of accident records.

March 2, 2010 · Tona · No Comments
Tags: , , , , , , , , ,  Â· Posted in: Government Revenue Scam, Police State, Red Light Cameras, Scamera Research, Scientific Facts, Speeding Ticket, Ticket Cameras

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