The convenient footbridge from Seattle’s First Avenue to the Washington State Ferries terminal closes Friday morning, sending walk-on ferry passengers to navigate a hillside and cross four-lane Alaskan Way for a few days.

Other detours for drivers and transit will take effect in the area next week.

The bridge closure allows demolition teams to remove a final piece of the Alaskan Way Viaduct over Marion Street, where the old footbridge is attached.

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A replacement steel bridge has been installed one block south, and state officials earlier guessed it would open in July. However, a rebuilt section of the Colman Dock terminal, where the steel bridge reaches the waterfront, isn’t ready yet.

“We hope to open the new terminal in a few days,” after final safety checks, ferries spokeswoman Hadley Rodero said.

Now that viaduct spans are nearly all eliminated in Pioneer Square, the state’s Highway 99 team gave demolition contractors the green light to tear down the section over Marion.

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Here’s a rundown of the traffic impacts of the demolition:

Walk-on ferry passengers

About 12,000 people per day walk onto ferries that travel between Colman Dock and Bainbridge Island or Bremerton, and a large proportion of those use the footbridge.

To walk from downtown to the ferry terminal, pedestrians starting Friday morning must cross surface Alaskan Way, until the new bridge opens. Ferry staff recommend walking from First Avenue down Madison Street to the waterfront. From the sidewalk level, pedestrians will take stairs, a ramp, or elevators up to the ferry-boarding area.

Driving

From Sept. 12 until Sept. 21, viaduct demolition at South Dearborn Street will close direct access between the Highway 99 tunnel’s Sodo interchange and First Avenue South.

But before the nine-day blockage, the Washington Department of Transportation will widen Alaskan Way South to provide four lanes along the waterfront — two in each direction — instead of demolition-related lane closures that now cause congestion.

Ferry customers from north Seattle who arrive via the tunnel will find normal routes to Colman Dock blocked. The ferry and tunnel teams haven’t drawn up detour-signage plans yet. One option would be to skip the tunnel and reach Alaskan Way via Belltown or downtown.

Buses, southbound

Starting Monday until early 2020, a dozen King County Metro bus routes serving West Seattle, White Center and Burien will leave downtown on diagonal Second Avenue Extension South (with a new bus stop at South Washington Street), which becomes Fourth Avenue South. Buses will take a westward swing at South Holgate Street or Edgar Martinez Drive, then onward to West Seattle.

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There are no added Sodo bus stops. This route change is meant to avoid delays of up to an hour from First Avenue South congestion.

Buses, northbound

From Sept. 12 to 21, buses going northbound into downtown will bypass the Dearborn demolition blockage by taking the Sodo Busway next to the light-rail corridor. During this nine-day detour, workers will re-stripe the right lane of northbound Highway 99 in Sodo for buses only, leaving two general-traffic lanes.

On Sept. 21, northbound buses will return to their previous route, which enters downtown through the Highway 99 Sodo exit and First Avenue South.