What is the difference between a deck level (デッキ) and a pedestrian overpass (歩道橋)?  How can we classify the pedestrian infrastructure at JR Ueno Station, pictured above?

The common characteristic: grade separation of motor vehicles and pedestrians.

I propose that deck levels have one primary characteristic which distinguishes them from pedestrian overpasses: direct connectivity to adjacent buildings.

In pedestrian overpasses, on the other hand, the connectivity is broken by staircases.  Moving from one building to another requires walking up one flight of stairs and down another.

Additional differences (generally resulting from the first characteristic):

• Deck levels are universally accessible (old-age friendly #高齢化).
• Deck levels are as convenient or more convenient than moving at ground level.
• Deck levels are frequently roofed; overpasses rarely are.  Examples include the JR station deck levels at Hamamatsucho, Shinagawa, and Osaki (浜松町・品川・大崎).
• Psychologically, a deck level feels like a “main” level, not a sideshow.  In some cases, the grade separated roads feel below-grade and peripheral.  This effect can be seen at Senri-Chuo (千里中央) and Tama Center (多摩センター).

Ueno Station’s aerial pedestrian structure is somewhere between deck level and overpass.   Ueno park itself can be accessed by ramp, and the south-east entrance has an elevator and escalators, but it appears that only four buildings have direct access (including the JR Ueno Station).  Most physical entrances are accessed by stairs.  However, since most journeys end or begin at the station, users of the structure rarely have to use stairs twice.