So, how exactly does a red light camera work, anyway?

Rather than capture images of tens of millions of cars passing by legally, the system is designed to predict which vehicles are most likely to run a red light. That’s because under the law, the camera system has to capture a driver entering the intersection after the light is red. So the camera must start recording before any violation takes place.

1. Running a red light

The system, which until March was operated by Redflex Traffic Systems Inc., uses sensors embedded in the pavement near the intersection to predict possible violations. When cars pass over the sensors at a certain speed, it triggers a video camera. The city’s new red light camera vendor -- Xerox State and Local Solutions Inc. -- is finalizing a citywide switch to an aboveground radar system.

A: A vehicle nears the intersection when the traffic light turns red.

B:If the vehicle is traveling at a certain speed, a video camera is triggered.

C:The video camera records the vehicle moving through the intersection with traffic light overhead, and a still camera takes a photo of the license plate (the flash).

2. Reviewing the data, issuing a ticket

Employees of the red light vendor review the videos and determine whether there was a violation. The data are forwarded to a second city contractor for further review. If a violation occurred, the second vendor sends the license plate number to state authorities to retrieve ownership information, and then the city issues a ticket by mail.

3. Once the ticket arrives

The driver receives a printed citation in the mail, which is also posted online along with the video and photos. The images can be accessed on the city’s website. Once at the site, a driver must enter the license plate number, citation number and city code (CHI) in the appropriate fields. The videos are often taken down after several months, and the city says it discards them after two years. But the data at the top of the still photographs remain. Example below:

Amber: The amount of time a yellow light lasts is called the amber time -- in this example 3.06 seconds. If the ticket indicates a time below city standards, right, a driver has grounds for an appeal. The city's standards are 3 seconds for 30 mph and lower and 4 seconds for 35 mph speed limits and higher.

Red: The amount of time a light was red at the time the photo was taken. In this example, the light had been red for 4.65 seconds.

Elapsed time: The elapsed time between the two photographs -- in this case 4.13 seconds -- is used to help determine the speed of the vehicle. If the vehicle covers more ground in less time, it is going faster.

4. Appealing the ticket

An appeal can be filed by mail or in person. Each gets assigned to one of about 80 contract lawyers hired by the city as administrative law judges.

Did city make its case: The lawyer uses a checklist to make a finding of whether the city has made its “prima facie” case, which essentially asks whether the city made any mistakes in issuing the ticket. Do the dates correspond? Is the location accurate? Does the vehicle make match the license plate? If there are any errors, the lawyer can throw out the ticket for lack of a prima facie case.

The evidence is challenged: The lawyer reads the appellant's case if the appeal is mailed, or listens to the appellant's arguments if in person. The lawyer watches the video and makes a finding about whether the "violation is factually inconsistent" with the evidence at hand. If the evidence does not support a finding that a violation occurred, the ticket is thrown out and the appellant is found to be not liable for the $100 fine.

Keeping a record: If the appeal was done by mail, the lawyer enters the information in the city's database, which includes an explanation of the reasons the ticket was thrown out. For in-person appeals, the city retains an audiotape of the hearing, which is public record.

SOURCES: Tribune reporting, City of Chicago