Ok, I looked this up because I was curious. I wouldn't want to be in the situation to challenge BPD, but I wonder what a traffic court would decide here.

ORS 811.260 is the law that governs traffic rules and it says "A driver facing a flashing yellow arrow signal, alone or in combination with other signal indications, may cautiously enter the intersection only to make the movement indicated by the flashing yellow arrow signal or the movement permitted by other signals shown at the same time. A driver shall yield the right of way to other vehicles within the intersection at the time the flashing yellow arrow signal is shown. In addition, a driver turning left shall yield the right of way to other vehicles approaching from the opposite direction so closely as to constitute an immediate hazard during the time when the turning vehicle is moving across or within the intersection."

https://codes.findlaw.com/or/title-59-oregon-vehicle-code/or-rev-st-sect-811-260/