Thresholds

BAC bathroom sign signifies that from that point forward access is meant for women only, mediates between the corridor meant for men and women and the bathroom meant for women only

BAC Stairwell, start point for a change in plane, mediates space between a floor and the floor below it

Change between natural earth and leaves, and man made pavement (although the 'natural' area was carefully arranged by man) the space mediates between the pavement meant for circulation and the area surrounding the tree which is meant to be looked at but not meant for foot traffic

Store front, I consider this a threshold not only because it mediates between indoor and outdoor space, but also because it has a significance that goes beyond that, usually only people with a certain taste/style/budget will cross this threshold

Threshold at MBTA train station, mediates between being able to access the trains and not being able to access the trains

also at an MBTA train station, the yellow strip mediates between space meant for circulation and space meant for the train, also meditates between a safe zone and a dangerous zone