New San Diego campaign aims to reduce traffic by replacing broken traffic sensors

Cars pass through an intersection controlled by traffic lights
“If a loop doesn’t work, it throws the timing off at an intersection,” said Jorge Riveros, the city’s transportation director.
(Howard Lipin / The San Diego Union-Tribune)

An estimated 10,000 in-ground magnetic sensors may be malfunctioning, making intersections less efficient


San Diego is launching a new campaign to reduce traffic congestion across the city by fixing thousands of in-ground vehicle detection magnets that help make intersections more efficient when they work correctly.

City crews will soon start taking a more proactive approach to fixing an estimated 10,000 of the city’s 30,000 in-ground magnetic loops that no longer work correctly because they either are worn out or have been damaged by construction, potholes or storms.

The loops detect vehicles and motorcycles stopped at an intersection’s outside edge and relay that information to computers controlling the intersection, so that red lights can be flipped to green.

Loops, a technology that dates to the early 1960s, can also be placed farther away from an intersection to sense when cars have begun to queue because of congestion. Information from those loops helps computers make stoplight timing at the intersection more efficient.

When loops — formally called inductive-loop traffic detectors — malfunction or fail to function at all, congestion on busy roadways gets worse than it needs to be. When loops in rural areas fail, drivers can sit at red lights for long periods even though nobody is driving on the cross street.

“When they’re not broken, they are typically 100 percent reliable,” said Jorge Riveros, the city’s transportation director. “If a loop doesn’t work, it throws the timing off at an intersection.”

The city budget for the new fiscal year, which began July 1, includes $332,000 for a three-person crew that will start the process of repairing all the malfunctioning loops in the city.

“We want to be more proactive and get our teams out there to make sure our loops are functioning,” Riveros said.

Loops only get fixed now when the city gets complaints that an intersection’s timing is off and a broken loop turns out to be the culprit. The new crew will include a traffic signal technician to diagnose the malfunction and two utility workers to fix the loop.

Fixes are relatively easy because the loops are low-technology devices.

“It’s basically just a wire or magnet that goes under the ground, and it works like a metal detector,” Riveros said. “They’re very sensitive, so we can pick up bicycles on the street in some cases. And you can dial the sensitivity up or down.”

But if officials make a loop too sensitive, it could get triggered by someone walking across the street with a lot of coins in their pocket.

A loop is typically 8 feet to 10 feet in diameter. Loops at the edge of an intersection are called presence loops, while those farther away from intersections are called pulse loops.

Of San Diego’s 1,650 intersections with traffic signals, only about 200 have no loops. That’s because they are in heavily urban areas like downtown, Hillcrest and Ocean Beach where stoplight timing is fixed.

“In dense urban areas with tight blocks, you use fixed timing because your cycle through the area is fixed,” Riveros said. “You have the signals moving in the same way every day at different times of the day, so you are not relying on detection to pick up vehicles.”

The new crew will tackle the city’s other 1,450 intersections, where Riveros said there are more than 30,000 loops. He said a very rough estimate of how many are broken is about one-third, but he stressed that estimate could be significantly off.

Officials estimate the new crew could fix 250 broken loops per year. At that rate, it would take them 40 years if it’s correct that 10,000 loops are malfunctioning.

Riveros said more crews could be added. He also said some fixes are diagnostic and don’t require digging, making them more efficient.

“A repair could take anywhere from a few hours to a few days, depending on what’s going on,” he said. “Fixing 250 per year is not putting a major dent, but it’s a dent we didn’t have before.”

Riveros said he plans to ask the city’s Performance and Analytics Department to help him figure out the most efficient way to begin tackling the problem.

The loops also boost the efficiency of adaptive signal timing systems San Diego has been adding to congested roads in recent years, Riveros said.
The city has added adaptive systems to Rosecrans Street, Mira Mesa Boulevard, Lusk Boulevard, La Jolla Parkway, Sorrento Valley Road and Friars Road near Fashion Valley mall.

They adjust the timing of green and red lights based on unpredictable events that have suddenly altered traffic flow, such as patrons from a sold-out movie flowing from a theater after the show.

When one stoplight sees an unusually high volume of cars, it immediately relays that information to each stoplight downstream in the series. That allows those stoplights to stay green for longer than they usually would at that time of day.