In the busiest hour of Seattle’s afternoon commute, 80 buses move through the Downtown Seattle Transit Tunnel.

Over a four-hour stretch, tunnel buses carry 15,000 riders.

Related: Who should pay to fix the Eastside traffic?

On Thursday, city and county officials said those buses could move out of the tunnel to make way for expanded light rail as soon as September 2018.

It could happen a year earlier than planned in part because of the project to expand the Washington State Convention Center.

City officials say that if nothing is done, extra gridlock could add an average of three and a half minutes to each bus trip, and seven minutes to each car trip through downtown.

“Doing nothing is not an option. That’s the challenge we’re trying to avoid. Lots of buses coming up on the surface street with no plan,” said Jon Scholes, CEO of the Downtown Seattle Association.

The DSA is working with the city, county and Sound Transit to come up with ideas for major changes to downtown traffic patterns to lessen the negative impact.

One idea is to convert a lane on both Fourth and Fifth Avenues to transit only.

Another is to make Fifth Avenue a two-way route for buses, and Sixth Avenue a two-way route for cars.

That scenario would emphasize separating buses and general traffic for safety and efficiency.

Buses now on Fourth and Second Avenues would move to Third and Fifth Avenues.

On First Avenue, a new streetcar will get a dedicated lane each way, with a second lane for general traffic.

Scott Kubly, the director of the Seattle Department of Transportation, says these changes could maintain Downtown’s present capacity for cars.

“That’s what we’re shooting for. And our modeling results show we have some really promising ways to get there,” Kubly said.

The overall plan includes improving traffic signal timing, new connections for bicyclists and feeding bus routes into the light rail system outside of downtown, which means passengers would have to transfer, but might get more frequent and reliable bus service.

The city is still considering the options.

Formal recommendations could come in March, with final decisions arriving a year from now.