The city maintains an page with background on the red light cameras. In it, the city claims:
Analyses of Chicago’s red-light equipped intersections conducted by CDOT found that dangerous angle crashes were reduced by an average of nearly 30% when a high angle crash rate intersection was equipped with red-light cameras. Rear-end crashes were found to increase on average at red-light camera equipped intersections. Rear-end crashes are more likely to result in minor injuries or property damage. The safety goal of the red-light program remains focused on reducing the most dangerous crashes. Rear-end crashes tend to decrease in frequency as driver behavior changes over time to comply with the red-light traffic laws.
The first study looks at 50 intersections where cameras were installed between 2006 and 2008. The study then compares the accidents which occured two years before the cameras were installed and two years after the cameras were installed. This study finds accidents dropped 9.8%.
The second study looks at 106 intersections where cameras were installed between 2006 to 2008. The study then compares the accident rate in 2005 compared to the accident rate in 2010 for the intersections. This study finds accidents dropped 8.22%.
Lets look at the second study first. It appears the red light cameras are making a difference. However, if we think about it, how do we know accidents dropped because of the red light cameras. What if there was another factor, say people driving less, that could account for the drop? To control for this, we need to know what the general trend was for accidents in Chicago. In 2005 there was 119,133 accidents and by 2010 this dropped to 80,922 acccidents, so reduction of 32%! Once we take this into account, we realize that actually intersections with red light cameras became more dangerous relative to the rest of the city.
So at this point, it should be obvious that either the red light cameras dramatically increase accidents or there is something wrong with the data. In this case, its a problem with the data. Starting in 2009, Illinois changed the definition of an accident from $500 to $1500 in damage. By increasing the dollar amount, this is going to reduce what qualifies as an accident. In fact, between 2008 and 2009, accicents dropped 36%, which is largely due to the change in the definition of an accident.
The flaws with the first study should also become obvious now. Since it relies on data from 2009 and 2010, that data is going to be significantly biased towards a reduction in accidents. As a result, you can’t compare the raw numbers after 2009 to prior years. This is why my study did not include 2009 data. I am currently working on another study, but that will only use data after 2009.
The bottom line is the Chicago's DOT analysis is useless. Its comparing apples to oranges, since the definition of accidents changed during the time period of the analysis. If the city really believes this is an accurate assessment of the red light cameras, then it shows red light cameras are actually a significant danger because accidents have only fallen 8%, while accidents in the city dropped 36%.