Followup: Why the ‘low bridge’ was out of service for 2 1/2 hours

June 19, 2014 at 3:06 pm | In Transportation, West Seattle news | 3 Comments

Following up on last night’s 2 1/2-hour shutdown of the Spokane Street Swing Bridge, aka “the low bridge,” we asked SDOT today to explain what happened. From communications director Rick Sheridan:

At the end of an otherwise normal opening, the bridge’s computer control system detected a fault in the docking process and stopped the closure. In the last 15 inches of docking we use a linear position transducer called a “temposonic,” which is very accurate and can measure movement to a fraction of an inch.

During the bridge closure yesterday afternoon, the temposonic reported that the moveable span’s position was incorrect. As the computer control system did not know if there was a malfunction or if the span was about to collide with the concrete pier, it stopped the span’s movement.

We deployed a Roadway Structures electrician who determined that the bridge’s span was properly aligned for docking and, after troubleshooting, concluded that the fault was with the temposonic. He reset the device, tested it and then restored the bridge to normal operations. We will monitor that device closely over the next several days to confirm that it is operating normally.


  1. “Have you tried rebooting your temposonic?”

    Comment by Jason — 9:17 pm June 19, 2014 #

  2. Isn’t rebooting the first thing you try?

    Comment by Charlie — 9:23 pm June 19, 2014 #

  3. The operator did the correct thing considering the potential damage that could have happened if the sensor was actually detecting an alignment issue. SOP is to call for a tech. 2 1/2 hours is commendable IMO.

    Comment by Born on Alki — 11:32 am June 20, 2014 #

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