I passed your comments along to our engineering team and this is the explanation I received:
“The reason the pedestrian pushbutton is required at these intersections is that if no pedestrian phase is required, the traffic signal will default to providing the shortest necessary green time for the vehicles on the minor street in order to minimize the major street delay. This default minor street green time may not be enough time for pedestrians to safely cross the street, thus the green pedestrian signal is only displayed if a pedestrian phase is called. At the intersections mentioned, the trail crossing follows the minor street (Four Mile Run Dr. at Shirlington Rd., Four Mile Run Dr. at Columbia Pike, etc.) therefore the pushbutton is required to provide the additional green time for pedestrians to cross.
This type of signal control (requiring the pedestrian phase to be activated) is consistent throughout most of Arlington outside of the Rosslyn-Ballston corridor. Pedestrians, and cyclists who are crossing as pedestrians, should only cross when a green pedestrian signal is displayed regardless of what is shown on the overhead traffic signal.”
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