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The traffic signals along Factoria Boulevard in Bellevue, Washington, generally don’ t flash the same length of green twice in a row, especially at rush hour. At 9：30 am, the full red/yellow/green signal cycle might be 140 seconds. By 9：33 am, a burst of additional traffic might push it to 145 seconds. Less traffic at 9：37 am could push it down to 135. Just like the traffic itself, the timing of the signals changes.

That is by design. Bellevue, a fast-growing city, just east of Seattle, uses a system that is gaining popularity around the US：intersection（十字路口） signals that can adjust in real time to traffic conditions. These lights, known as adaptive signals, have led to significant declines in both the trouble and cost of travels between work and home.

“Adaptive signals can make sure that the traffic demand that is there is being addressed,” says Alex Stevanovic, a researcher at Florida Atlantic University.

For all of Bellevue’ s success, adaptive signals are not a cure-all for jammed roadways. Kevin Balke, a research engineer at the Texas A&M University Transportation Institute, says that while smart lights can be particularly beneficial for some cities, others are so jammed that only a sharp reduction in the number of cars on the road will make a meaningful difference. “It’s not going to fix everything, but adaptive signals have some benefits for smaller cities,” he says.

In Bellevue, the switch to adaptive signals has been a lesson in the value of welcoming new approaches. In the past, there was often an automatic reaction to increased traffic: just widen the roads, says Mark Poch, the Bellevue Transportation Department’s traffic engineering manager. Now he hopes that other cities will consider making their streets run smarter instead of just making them bigger.