Old Narrows Bridge ruts explained

The in basket: I’ve been noticing a series of patched ruts on the old Tacoma Narrows Bridge that are oddly clustered and I wondered about their cause.

They are in the center all-purpose lane, up against the grate that separates it from the number 3 lane.

They stop midway across the bridge and don’t affect any other part of that lane.

I wondered what caused them.

The out basket: Claudia Bingham-Baker of the state’s Olympic Region for state highways says they are caused by traffic, which is heaviest in that lane.

“On the 1950 Tacoma Narrows Bridge,” she said, “the far right lane is what we call an ‘add/drop’ lane. The lane adds at Jackson Avenue, continues across the bridge, and drops at the 24th Street exit.”

Since changing lane on the bridge is prohibited, many drivers who otherwise might use the outside lane but know they’ll have to move left at the other end of the bridge, use the second lane instead. Notably, that’s true of large trucks.

“The adjacent, second lane, tends to be the primary lane motorists use when crossing the bridge,” Claudia said.  “The potholes you see are caused by the higher volumes of traffic and trucks using that primary second lane.

“We have plans to resurface the bridge deck in the 2015-2017 biennium, and will do our best to keep the potholes patched until then,” she said.

The reason the damage in that lane stops half-way across the bridge is less certain.

“There’s some speculation that because the bridge is arched,” she said, “the cars going over the first half of the bridge are on an incline and pushing against the pavement; going downhill from the crest they are accelerating less and causing less stress on pavement. Just speculation.”

 

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