i hate to burst everyone's bubble, but that tree might not be (merely) a decoration.
construction crews put trees at the highest elevations of projects at milestones, or if no one is injured during construction.
In building construction, topping out (sometimes incorrectly referred to as topping off) is a ceremony held when the last beam is placed at the top of a building. The term may also refer to the overall completion of the building's structure, or an intermediate point, such as when the roof is dried in. A topping out ceremony is usually held to commemorate the event.
While common in the United Kingdom, Germany, Czech Republic, Slovakia, and Poland, the origins of the ceremony are obscure. Its practice has long been an important component of timber frame building. This tradition migrated to America with European craftspeople. A tree or leafy branch is placed on the topmost beam, often with flags and streamers tied to it.
during the construction boom between the late 1990's and mid-2000's, evergreens adorned a lot of new high-rises in seattle.
if you read the link above, this might have been a practice carried over from old-world europe.
my first thought when i saw that tree on the viaduct was that no one had been injured on the job.
then again, it really could be some festive construction workers.