It’s big, it’s gray, it’s ugly — it’s physics

Diagonal “bridges” are a lot of metal for a simple traffic signal. Size of intersection is a reason.

It does seem a bit overkill.

My colleague Jim Davis passed me this question: “Why are there large metal bars in the air that crisscross Highway 9 in the Clearview and Maltby areas? Traffic signals are attached to the bars, but do they do anything else?”

An example is at the intersection of Highway 9 and Highway 524, where a massive gray metal pipe crosses the intersection diagonally. (Similar if smaller examples are found elsewhere in Snohomish County, too, like over Murphy’s Corner between Mill Creek and Everett’s Silver Lake.)

What are they hiding in there? A bunch of utility wires? A really short aqueduct? A secret passageway for mutant squirrels?

It comes down to math.

“The large, clunky bars to which Mr. Davis refers are called signal bridges,” Washington State Department of Transportation spokesman Tom Pearce said. “If you look at the intersection, including sidewalks, the right-of-way is about 120 feet across SR 9 and more than 100 feet across SR 524. Traffic signal regulations require a minimum and maximum distance from the stop line on the road to the traffic signal.

“It would be hard to place four traditional one-armed masts at each corner of the intersection that would meet these criteria. In addition, the arms reaching over SR 9 would need to be about 75 feet long to account for the double left-turn lanes, which would require some pretty beefy masts and arms.

“Aside from meeting the requirements, it’s more efficient to have one diagonal signal bridge that bears all of the traffic signals rather than four separate mast arms,” he said.

OK, but is there another hidden use for that cavernous bridge?

“The signal bridge only has wiring for the signals in it,” Pearce said.

Well, I’m still going to imagine mutant squirrels.

Nothing’s too squirrelly for Street Smarts. Have a question or topic of your own? Email or call 425-339-3432.

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