New, adaptive traffic lights on Mercer Street — the first of their kind in Seattle — have sensors to detect congestion in real time and then adjust their timing accordingly.

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New, reactive traffic signals along the Mercer Street corridor in Seattle have made for more reliable and, for many drivers, quicker daily commutes, the Seattle Department of Transportation announced Monday.

About three weeks ago, 32 intersections in the Mercer corridor began using adaptive traffic lights that use sensors to get a real-time picture of traffic conditions and then adjust their timing accordingly.

Mercer Street, where about 60,000 vehicles a day navigate the infamously messy traffic between Interstate 5 and Lower Queen Anne, is the first spot in the city to get the new traffic lights.

Instead of changing from red to green based on prearranged timing — say 30 seconds for east-west traffic, followed by 20 seconds for north-south traffic — the new system reads where congestion is coming from in real time and then automatically adjusts its timing.

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In total, 18 intersections on Mercer have the new lights, as well as 10 on Roy Street, three on Republican Street and one on Westlake Avenue.

So far, the system is resulting in more reliable commutes in both the morning and the evening, headed both directions on Mercer, according to SDOT data.

Before the new system, which is known as SCOOT — split cycle offset optimization technique — it took about 18 minutes, on average, at the busiest traffic time of the day (around 6:30 p.m.) to traverse the length of Mercer Street to I-5. That number has since decreased by about three minutes.

But the bigger change has come not in the average commute times, but in the worst commute times. Previously, on the worst commute days — which happen about one weekday per month — it took more than 35 minutes to travel the 14 blocks from Queen Anne Avenue to I-5’s onramps. Now those extra-bad commutes take just over 20 minutes, according to SDOT.

SDOT has seen improvements in reliability of commutes, if not necessarily average times, in both directions on Mercer, both morning and evening.

“It’s people being better able to plan their lives and knowing that this trip normally takes 20 minutes, but I never have to plan for it going more than 30 minutes,” SDOT Director Scott Kubly said.

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The evening commute on Mercer east toward I-5, the busiest three-hour stretch for the boulevard, has seen reliability improve by 38 percent and average travel times decrease by 2.7 minutes.

Westbound commutes, in both the morning and the evening, have seen similar increases in reliability, meaning the worst commute days are not quite as bad as they used to be. But westbound trips have actually seen a slight increase in average travel times, about a minute for both morning and evening commutes.

Kubly said those increases were small enough to fall within the margin of error for the three-week sample.

With both directions taken into account, the average commuter will save just under 7.5 hours and 1.5 gallons of gas a year, according to SDOT.

Pedestrian crossings are factored into the adaptive traffic signals, but only when pedestrians push the button to call for a walk signal. Forget to push the button and you won’t get a walk signal.

The new system cost about $4 million, the capstone of a half-decade of changes and improvements to Mercer, which cost more than $260 million and brought two-way traffic, bike lanes and wider sidewalks, but not congestion relief.

Over the next several years, SDOT plans to roll out two more phases of adaptive traffic lights. The next set of new signals will go on Denny Way, between Western Avenue and I-5, followed by new signals at a smattering of intersections in Seattle Center and South Lake Union.