A green light for energy efficiency in Minneapolis

The switch from old, incandescent traffic signal bulbs to new, energy-efficient LED lights is saving millions in electricity costs for cities across the country, as well as reducing greenhouse gas emissions. A report to the Minneapolis City Council last week said the city has saved $2.2 million since it started converting the lights in 1997.

As the conversion to LED, or light-emitting diodes, nears completion in Minneapolis and elsewhere, though, some traffic engineers see reason for flashing-yellow caution.“The LEDs have more lumination output, what we call punch. They’re brighter,” said Don Sobania, traffic signal operations engineer for the city of Minneapolis.

In most cases, that’s a good thing. The lights are easier for drivers and pedestrians to see, compared to the dusty bulbs they replace. But on busy corridors with several stop lights one after another, the bright green lights can catch drivers’ eyes several blocks away, when the intersection they’re approaching might be red.

The effect is called draw-through, Sobania said, and it causes accidents even with the duller incandescent lights. Usually it happens when a driver is stopped at a red light at the front of an intersection. When the next intersection changes to green, an inattentive driver’s mind can sometimes tell the foot to put the pedal to the gas. The results can be nasty, Sobania said.

Some traffic engineers are worried that if the intensity of green lights increases, so too might the likelihood of drivers’ misinterpreting them.

All of the approximately 800 signaled intersections in Minneapolis have been equipped with red LEDs. More than 600 of those intersections also have green LEDs, but there’s an ongoing discussion about how to proceed with green lights at about 150 other intersections in downtown Minneapolis, Sobania said.

Otherwise, the conversion has been a no-brainer success for cities. The lights cost significantly more, about $40 per light instead of 70 cents. But even subtracting for costs, the city has saved in electricity costs. And the switch has kept an estimated 7,285 tons of carbon dioxide from the atmosphere.

Because they use less electricity, it’s also becoming feasible to install battery backup power at traffic signals so they remain on in the event of a power outage.

And the bulbs need replacing far less frequently than incandescents, which reduces maintenance costs. Red lights that were put in in 1997 and 1998 have yet to burn out

“We don’t know when it’s going to happen, because they just keep living on and on,” Sobania said. “They were supposed to last six years. Now they’re in their tenth year.”

Read the Minneapolis Department of Public Works report on LED Traffic Signal Conversion [pdf].
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