The Sun used that method to document erroneous speed readings from six city speed cameras. Because the city does not have lines painted at set intervals, markings such as lane dividers and cracks were used to measure how far a vehicle went over the split-second between the two photos.
The Sun also reported that a Mazda had been cited for speeding while stopped at a red light. Xerox State and Local Solutions, the city's previous contractor, acknowledged that that camera and four others had error rates of 5 percent, prompting the city to take them offline. Xerox said high-walled trucks played a role in causing "radar effects" that produced inaccurate readings.
Rawlings-Blake has defended the safety benefits of the city's speed camera program, which has generated more than $48 million in paid fines since 2009, amid the revelations that errors got through a multi-layer review meant to catch mistakes. The vendor does the initial review before sending citations to the police for authentication. Only if police approve a ticket can it be mailed to a motorist.
Speaking to reporters last month, Rawlings-Blake said "our focus, as always, is on safety," adding, "This is about making sure that our young people, who are going to and from school, have safe passage." Speed cameras are allowed in school zones and highway work zones; $40 tickets can be issued to vehicles clocked going at least 12 mph over the limit.
Maurice R. Nelson, managing director of Brekford, did not respond to requests for comment. But he has said his firm's cameras would bring marked improvement to speed enforcement in Baltimore.
"The old radar cameras have not progressed with technology," he said in December, adding that tracking technology can focus on and follow a specific car, reducing machine-created errors. "If you're using the old radar cameras and it's picking up something that's not the car in the photograph, you leave yourself open to errors."
While the city has described Brekford's cameras as "state of the art," Kang, of Hopkins, said tracking radar technology is more than 70 years old. "It's been around since before World War II," he said.
In basic terms, radar uses radio waves to determine an object's speed by calculating changes in frequency as the object moves toward or away from a transmitter. This change is known as the Doppler shift.
Other speed cameras, including those used in state highway work zones and in Baltimore County, use laser technology called "lidar." Instead of radio waves, lidar uses light pulses to gauge speed.
Though more accurate than radar, lidar can be thwarted by fog because it relies on light particles, said Kang, who has researched lidar. "That's the problem with lidar: You don't get 100 percent detection. But you will reduce the error. With radar, you will always get the signal, but the signal may be wrong."
"The question," he said, "is which poison do you want to take?"
Berra, the photonic engineer, said: "If everything is going well, both radar and lidar are extremely accurate. If you have ideal conditions, one car on the road, and the system calibrated, the error rate should be zero."
But by their very purpose, speed cameras are often put on major roads with lots of traffic. Davis said being outside for weeks or months on end can cause misalignment that can, in turn, impair accuracy.
While state law requires every speed camera to be calibrated to the manufacturer's specifications once a year, Davis says that's "way too long" between calibrations. And Kang said the only real way to test accuracy is to drive a vehicle past it at a known velocity, difficult in a city with 83 cameras.
Speed camera proponent David Kelly said in hindsight it might have been wise for the city to use tracking radar when it launched its speed program in 2009. But he's glad the switch is occurring now with a new vendor onboard.
"Now they do have a chance to start over and get it right from the beginning," said Kelly, executive director of the National Coalition for Safer Roads. "This is a real natural time to say, 'Let's make sure we've got this right.'"