On Tuesday, meters at the Mercer Street on-ramp to Interstate 5 will be switched on for the first time on a weekday.
Drivers wonder how the meters will affect traffic.
"Hoping that it's helpful rather than backing it up more," said Rebecca Aguon.
The meters will pause each car for a few seconds, forcing a more orderly merge.
Traffic engineer Morgan Balogh, of the Washington State Department of Transportation, said southbound I-5 through Downtown Seattle averages 7,000 vehicles per hour in the afternoon.
If all goes well, metered ramps can help the freeway absorb 3 to 5 percent more cars.
But that probably won't happen at the peak of the afternoon commute.
"We should see some benefits, say 2 to 3:30 (p.m.) and 6:30 to 8 (p.m.)," Balogh said.
Between 3:30 and 6:30 p.m., when traffic is worst, WSDOT doesn't expect any benefit on I-5 at all.
That's because traffic is so congested at that time, there's not much that can be done.
For drivers concerned about an extra light making the backup on Mercer Street even worse than it is now, Balogh has a reassuring message: "This project won't make the Mercer corridor any worse."
The Seattle Department of Transportation is in charge of Mercer Street, and spokesman Norm Mah said engineers will be watching from the traffic center, ready to adjust signals.
"If there's any kind of issues that arise, we'll make any adjustments we need to, to minimize the impact," Mah said.
The state has been testing ramp meters on the weekends, and pushed back the weekday rollout until Tuesday after drivers didn't realize they could use both southbound lanes.
WSDOT has since added more signs.
The Seattle area already has 245 ramp meters, and WSDOT officials say they typically reduce collisions on freeways by one-third.
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