The revenue stream tempts governments to reduce the yellow interval, which then decreases safety: http://www.copradar.com/redlight/

The part about how they are a complete scam? http://blog.motorists.org/illinois-red-light-cameras-scam/

I first learned about automobile tracking from red light cameras here in Montana.

A company called RedFlex installed cameras here. They subsidized the installation cost in exchange for a cut of tickets.

RedFlex admitted to bribing city officials to get them to install cameras.

Safety measure or not, red light cameras are a joke. Chicago's red light cam program is a failure.

Edit: I have to agree with the other commenter though - red light cams were meant to bring in revenue to the city of Chicago. They bought into the program by outsourcing to a third party company (big shock, most of chicago looks like the poster child for a corporation owned city). They're not making the money they expected on the deal though.