admits
May a pedestrian step onto the crossing now?
Asked of every arrival, and again of whoever is still queued each time the crossing's state changes. Answering false does not end a turn that is already open -- people already on the crossing finish and leave -- it only stops more from joining, which is how a turn is brought to a close without evicting anybody.
Parameters
crossing
the crossing being asked, which reports how many are on it, how many wait, and how long the vehicle at the head of the queue has been waiting