Tomorrow night at 10 pm, the onramps to the Alaskan Way Viaduct/Highway 99 start closing. Within two hours – by 12:01 am Friday – the entire stretch of 99 from the West Seattle Bridge to the Battery Street Tunnel will be closed. More vehices will be on the “low bridge” – officially, the South Spokane Street Swing Bridge – than usual, both bicycles and pedestrians, which have no other nearby path across the Duwamish River, and motor vehicles, because of detours.

(2015 photo by Don Brubeck – bicycle riders waiting while barge goes through opened ‘low-bridge’)

The “low bridge” will continue to be opened for vessel traffic. One difference from the 2011 weeklong Highway 99 shutdown: Those openings are now communicated via Twitter, @SDOTBridges. But the tweets tend to be sent just as the bridge starts to open, meaning they’re no help with advance planning, if you’re leaving your residence and wondering if an opening is imminent.

With that in mind, a WSB commenter wondered if there is any way for SDOT to tweet sooner about impending bridge openings. We asked SDOT communications director Rick Sheridan, who talked with the Bridge Division and replied:

We are required by federal law to open a bridge when a boat makes a request for an opening. Typically, we have less than three minutes notice from the boat making its request to an opening occurring. Part of the opening process is to alert our Transportation Operation Center, which then puts the information out via Twitter.

The presence of a boat on a nearby waterway does not necessarily mean that an opening is required. Boats will sometimes stop prior to the Ship Canal bridges or the lower Spokane Street Swing Bridge and not request an opening at all.

We will continue to tweet notices of bridge openings as soon as they are requested and look for opportunities to speed up the transmission of that information.

We’ll be adding the bridge-opening tweets to our expanded commute coverage during the Viaduct closure. You can also find the link any time on the WSB Traffic page (which will have some renovations before Friday).