Smart Cameras Blog

Part 1 of this report considered accident trends in Chicago between 2001 and 2008. This part focuses on accidents at traffic signals. The theory behind red light cameras is that people will drive more carefully at intersections. If they are driving more carefully, we should expect accidents to fall across all traffic intersections, not just the ones with red light cameras.

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.

rshah Chicago, Red Light Cameras