A two-way, low-speed bicycle lane, protected from other traffic, would be carved out of Fourth Avenue in downtown Seattle under new concepts revealed Thursday night by local transportation planners.

The lane would be on the west side of the street, and planters, posts or curbs could divide bicycles from motorized traffic heading north.

Currently, Fourth Avenue is a bicycle gantlet. A thin, striped lane near City Hall fades away near Westlake Park, where riders speed down the small hill and merge with cars, to the frustration of both drivers and cyclists.

This new route, a piece of a broader “One Center City” plan to manage a pending traffic crisis, would resemble the existing Second Avenue bicycle lane.

But how would the bikeway affect cars and buses?

The left car lane would be a combination of curbside parking and left-turns only, on a green arrow, like on Second, said Eric Tweit, a senior planner for the Seattle Department of Transportation.

The city and King County Metro Transit earlier negotiated about whether to put a second bus lane on Fourth, to help meet growing demand. But they can’t shove both a bike lane and a second bus lane onto Fourth without ruining general traffic.

So as an alternative, Metro would shift its Fourth Avenue buses to Fifth Avenue, where the northbound bus lane would be stretched a couple blocks north — and then jog over to Sixth Avenue to continue north toward Interstate 5 entrances. Routes 74, 76, 77, 301, 308, 311 and 316 would make the shift, totaling 28 buses weekdays from 4:30 p.m. to 5:30 p.m.

However, Community Transit and Sound Transit north-side commuter buses, which carry about 15,000 daily riders, would stay on Fourth, where there would be three fewer bus stops overall.

Buses would need to leapfrog each other less often, and presumably can do without a second bus lane, explained Tom Brennan, consultant with Portland-based Nelson Nygaard. Bus trips on Fourth are supposed to improve from 10 minutes to just under 9 minutes, a consultant chart predicts.