The change is meant to help mitigate the upcoming traffic disaster when the Alaskan Way Viaduct shuts down. About 100,000 people board or get off a bus on Third Avenue every weekday.

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Hey, you, quit driving on Third Avenue. Unless you’re driving a bus. Or a delivery truck. Or it’s nighttime.

Some limited exceptions remain, but beginning this week, Seattle’s pre-eminent downtown transit corridor is closed to cars every day of the week, from 6 a.m. to 7 p.m.

The changes had been planned for this fall, but they were bumped up by about a month as the city tries to deal with the coming closure of the Alaskan Way Viaduct. The viaduct will be closed for three weeks before the new Highway 99 tunnel opens later this fall, robbing downtown of an arterial that carries 85,000 cars a day.

“One key pillar of our plan for dealing with the traffic impacts is investing in transit and working to get Third Avenue to work as efficiently as possible,” wrote Jeanne Clark, a Seattle Department of Transportation spokeswoman.

Third Avenue between Stewart and South Washington streets has been bus-only during morning and evening rush hours for more than a decade, beginning in 2005, when the downtown bus tunnel closed for two years to be retrofitted for light rail.

But until this week, cars could still drive on Third in the middle of the day and on weekends. That’s no longer the case.

And even at night when cars are allowed on Third, all left turns are now prohibited from the avenue.

King County Metro, which runs the buses, says Third Avenue is “the busiest transit corridor in North America.” About 2,500 buses travel along Third each weekday, picking up or dropping off 100,000 riders, according to Metro.

Bikes will continue to be allowed on Third Avenue, and commercial deliveries will be allowed between 9 a.m. and 3 p.m.

Detective Patrick Michaud, a Seattle Police spokesman, said traffic officers would be out on Third to enforce the change, beginning with an “education phase.”

If you’re pulled over for driving on Third, you could get a $136 ticket, but in the first few weeks it’s more likely you’ll get a warning, Michaud said.

“We want to get people educated before we begin enforcement,” Michaud said. “It’s going to be some growing pains for everybody.”

Metro is also installing new ORCA card readers, so they’ll be at every bus stop on Third. Construction on the card readers, which allow faster boarding because riders can pay on the street and enter through any door, should begin in November and be complete by March 2019, SDOT said.