New San Diego campaign aims to reduce traffic congestion by fixing broken sensors

San Diego will have a crew start repairing thousands of malfunctioning in-ground vehicle detection magnets.
San Diego will have a crew start repairing thousands of malfunctioning in-ground vehicle detection magnets that are designed to help make intersections more efficient.
(The San Diego Union-Tribune)

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


The city of San Diego is launching a new campaign to try 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 properly.

Workers soon will 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 red lights can be flipped to green.

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

The loops also boost the efficiency of adaptive signal timing systems that San Diego has been adding to congested roads in recent years, said Jorge Riveros, the city’s transportation director.

The city has added adaptive systems to Rosecrans Street, La Jolla Parkway, Mira Mesa Boulevard, Lusk Boulevard, 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 flowing out of a theater after the show.

When one stoplight senses 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 longer than they usually would at that time of day.

When loops — formally called inductive-loop traffic detectors — malfunction or fail to function at all, congestion on busy roadways gets worse. 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,” Riveros said. “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.

Cost thresholds requiring City Council approval would be raised as the city enters an era of significantly more infrastructure spending.

July 29, 2022

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.”

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

Of San Diego’s 1,650 intersections with traffic signals, all but about 200 have loops. The ones that don’t 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 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 one-third, but he emphasized that estimate could be off significantly.

Officials estimate the crew could fix 250 broken loops per year. At that rate, it would take 40 years to fix 10,000 malfunctioning loops.

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 & Analytics Department to help him figure out the most efficient way to begin tackling the problem.


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