Naperville is going to stop using red light cameras for ticketing right turns (at most intersections). This is a very sensible policy, since right turn traffic violations are usually not a significant safety risk (especially compared to people running straight through red lights). It’s good to see a city treat its citizens fairly and not find a sneaky way to extract some money out of them.

This part analyzes accident data obtained from the city of Chicago. The city’s study looks at 20 RLC intersections and 20 control intersections and compares them. The control intersections are supposed to be similar intersections to the RLC intersections in terms of traffic and accident rates. By using control intersections, it is possible to isolate the effect of the red light cameras from other factors affecting accidents, e.g., people driving less.

The study compares the accidents 1 year prior to the startup date to accidents 1 year after the start up date for the red light cameras. In evaluating the study, we had to eliminate 10 of the intersections, both control and RLC, because they did not have a full 12 months of prior data. This left us with 10 control intersections and 10 RLC intersections.

The control group experienced a 3.8% decrease in accidents, while the RLC group experienced a 5.3% decrease in accidents. In sum, there was only a 1.5% different attributable to the RLCs. This suggests a very small benefit for the RLCs. This decrease is a much smaller figure than the city claims.

This study was conducted entirely by the city of Chicago. The report is dated October 2007. The departments involved were the OEMC and the Traffic Management Authority. The study was not made public. I received the data after a FOIA request that required the assistance of the Illinois ACLU to push the city to release this information.

I cannot vouch for the accuracy of this study. The city picked the camera locations to study. I have no idea if they tried to “massage” the data to gain beneficial results. The city is planning a followup report due later this summer. As far as I know, they do not having any impartial experts involved in the process. I hope the city’s analysis will be fair, rather than manipulating the data to justify the effectiveness of the red light cameras.

The results show that accidents went UP 5% at the intersections with RLCs. This is a shocking result, because you would expect accidents to decline with the introduction of RLCs. The results also stand counter to the general trend of accidents falling at in Chicago about 7% between 2005 to 2008, as well as the 7% drop of accidents at traffic signals.

Results:

The simplest analysis we did was to look at accidents 1 year prior to the startup date and compare it to accidents 1 year after the start up date for the red light cameras. We found a total of 1118 accidents at the 39 intersections 1 year prior to the startup of the RLC. We compared this to 1 year after the startup date and we found a total of 1192 accidents. So in first year the RLCs were used, accidents as a whole went up at the RLC intersections!

We also used a longer time period, which varied according to the data we have for the individual intersection. The longer time period varied from 347 days to 1080 days with a mean of 500 days. This data also found an increase 4.5% in the number of accidents.

We examined the data to see if there was any “bumps” or changes in red light accidents over time. For example, was there an immediate decrease in accidents and then accidents went back up after 3 months? We found that the rate of accidents was constant at intervals of 1, 3, 6, 9, and 12 months.

Unfortunately, we don’t have access to the entire data set (without manually entering in the Tribune data), so we can’t perform a more sophisticated analysis, such as whether there were changes in the types of accidents (angle & rear end) at RLC intersections.

There are two sources of data on accidents at the RLC intersections. The first is from the Illinois Department of Transportation (IDOT), which is used here. The second is from Chicago’s Department of Transportation (CDOT). Accidents at intersections are measured differently by both agencies. CDOT defines the intersection as extending along the arterial street for a distance of 25 to 50 feet while IDOT will use measurements of 150 to 300 feet. Despite the differences in measurements, we believe any tanglible affect of RLCs should be found at either distance.

The startup dates for the cameras varied from January 1, 2006 to Nov 26, 2007. The analysis compares accidents in the year prior to the installation date to one year after the installation, e.g., for a camera installed on April 1, 2006 the year prior data is from March 31, 2005 to March 31, 2006. The year after is April 1, 2006 to April 1, 2007.

Here are the main points:

1. Accidents at traffic signals are down, but once we account for the general drop in accidents, the accident rate at traffic signals is holding relatively constant. (There is no halo safety effect from the red light cameras on accidents.)

2. There appears to be a steady drop in Angle crashes over the last 5 years, indicating that there is a change in behavior at traffic signals.

First, consider the overall trend in accidents at traffic signals:

A quick look at this graph shows that it is very similar to the overall drop in accidents. To factor out the general trend of accidents dropping, we next determined the percentage of accidents that occur at traffic signals in Chicago between 2001 and 2008.

This graph shows that the percentage of accidents at traffic signals hasn’t changed appreciably between 2001 and 2008. If people were driving more carefully at traffic signals, it would be expected that accidents at traffic signals would become relatively rarer. For example, dropping from a share of 25% of all accidents to 20% of all accidents. The results here suggest that traffic signal accidents are holding relatively constant. This also suggests the RLCs are not having a halo effect because accidents are not dropping throughout the city at traffic signals.

The next step was to consider the types of accidents occurring at traffic signals in Chicago between 2001 and 2008.

The biggest finding here is the drop in Angle crashes (crashes where cars are heading in different directions). Between 2003 and 2008, there is a 5% drop in Angle crashes at traffic signals. This is a large! This drop supports the argument that red light cameras are leading drivers to be more careful at traffic intersections.

For both turning and rear end type crashes, their was little change with the exception of the 2008 numbers. Hopefully, 2009 numbers will identify if there is a trend in an increased share of rear end accidents and a decreased share of turning accidents at traffic signals.

This data was obtained from the Illinois Department of Transportation (IDOT). IDOT has a separate variable for traffic signals in their accident data.  This allows us to isolate accidents that occur at traffic signals.

The two important points:

1. Accidents are down 21% between 2001 and 2008, largely due to people driving less.

2. Any analysis of accidents needs to account for this trend. This trend suggests fewer accidents at construction zones, school zones, highways, and traffic signals (EVERYWHERE). If you are suggesting anything has contributed to safety, e.g., red light cameras, you need a control group to identify the contribution of the safety device or program. Otherwise it will appear that red light cameras have led to a drop of 20% in accidents (when the drop is due to other factors).

Lets begin with a graph of accidents in Chicago between 2001 and 2008:

Clearly there has been a decline in accidents (over 21% between 2001 and 2008). There are a lot of factors affecting accidents from gas prices to the weather. One factor that can be taken into account is how much people drive. The IDOT provides an Annual Vehicle Miles of Travel statistics that allows us to account for how much people drive over a year.

The graphs shows that vehicle miles have dropped in Chicago from 12.6 billion miles in 2002 to 11.6 billion miles in 2008. If people drive fewer miles, it seems reasonable that there will be fewer accidents. We can analyze this by looking at the accident rate for each mile traveled.

This graph shows when we take into account the fewer miles driven in the city, there is no longer a steady drop in accidents. Instead, it’s hard to find a significant trend between 2001 and 2008. It’s also important to note that you can find either a trend of accidents going up, down, or holding steady depending on the what years you include. So once we account for people driving less, there is not a steady trend of accidents dropping.

A longer description can be found at The Expired Meter. I will provide the full report in a few days for everyone.

Download the KML here

UPDATE: Here is a link to the complete table with all the revenue information

Here is a link to the KML data, please let me know if anything needs to be updated.

Now that legislators have seen how the cameras are used, they are not as supportive:

The chief author of the 2006 Illinois law that green-lighted suburban red-light cameras is now pushing a legislative revision that could effectively undo his original bill. The proposal, from Rep. Angelo “Skip” Saviano, R-Elmwood Park, would ban the use of cameras to ticket motorists for rolling right turns on red, a significant limitation that would strip away the financial incentive for municipalities to install the devices.

By some estimates, up to 90 percent of infractions flagged by cameras involve failures to make proper stops before turning right on a red light. While illegal, such maneuvers rarely lead to serious accidents.

. . .

But Saviano said the practical impact of the cameras has become distorted. “It went from a safety issue to a revenue issue,” he said. “The bottom line is people can’t afford to pay the gosh darn fines. They are grumbling.”

Saviano’s measure is one of several proposals to rein in the use of red-light cameras that have been filed by Illinois lawmakers in anticipation of the legislature’s 2010 session.

I am hoping to review this data in a more detailed post later. But for now, here are some important snippets:

Cameras are said to reduce accidents, but collision records compiled by the Illinois Department of Transportation indicate that accidents increased at many city intersections the year after red-light cameras were installed. In fact slightly more intersections saw an increase than a decrease, the data show. The city tells a very different story. Crash statistics compiled by the city reflect broad success in reducing accidents with cameras, and the city could not explain why the numbers are so different.

The read from the state numbers is this: Although some Chicago intersections indeed appear to benefit from the presence of cameras, nearly 60 percent do not.

By year’s end red-light cameras will be installed at 189 Chicago intersections, the most of any big U.S. city. Sprawling Los Angeles, where the car is king, has 32; New York, 150.

This work casts doubt on the city’s claim that the cameras reduce accidents. (If the cameras don’t reduce accidents, then they aren’t providing the promised safety benefits.) Read the city’s claims here:

The City of Chicago reports crashes have been reduced by 20 percent in the two years since the camera technology was installed in early 2006 at 10 intersections. . . . Crashes decreased 30 percent, from 1,055 in 2004 to 736 last year, at intersections where red-light cameras were installed in 2004 and 2006, according to the Chicago Office of Emergency Management and Communications.