I've chosen to say "model", not "simulation", because no checks have been done on these :
"What the bridge user sees" (to an approximation!)
The bridge user sees the planar deck, the top-surface of the following image - stretching away for the length of the bridge and across the width of the passageway.
The structure beneath is hidden, in reality, from the bridge-user.
The bridge-deck is modelled and displayed "upside-down".
(R. Smith, 06Jun2016)