Nighttime closures of Seattle's northbound SR 99 Duwamish River Bridge begin on July 12 and we'll have 14 nights of closures for work to preserve the structure also known as the First Avenue South bridge. Because this is the main detour for the West Seattle high bridge, those who travel it will need to plan ahead. The lower West Seattle Bridge will be open to all traffic on nights at the First Avenue bridge is closed. Other options include the South Park Bridge, Tukwila International Boulevard,