@AestheticsInMotion
If I had to guess, it's there because of confusion, and it was originally intended to improved the traffic flow. How could this improve the flow? Previously, there was just the right-turn arrow, which would often be green even when the through light was red.
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However, Seattle drivers can be clueless. My guess is that when the through light and turn arrow were both red, drivers wanting to turn right were backing up behind somebody who though that the red turn arrow meant that you had to wait for it to turn green. The sign was probably added to let drivers know that right-turn-on-red is allowed at the intersection.
A similar sign was added in my neighborhood in an attempt to reduce backups from Highlands drive onto Federal.
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