歩道橋 and 横断歩道: an overpass and a street-level crossing
Compare viewing a road from an overpass with waiting before a crosswalk, then identify which crossing route was actually used.
A pedestrian bridge above a road
歩道橋, read ほどうきょう, means a pedestrian overpass. 歩道橋の上から道路を見ました。 says that someone viewed the road from the top of the overpass.
歩道橋の階段を上がりました。 says that someone went up the overpass stairs. Going up does not itself establish a completed crossing to the other side.
A crossing at road level
横断歩道, read おうだんほどう, means crosswalk. 横断歩道の手前で待ちました。 says that someone waited before reaching the crosswalk.
歩道から横断歩道へ進みました。 says that someone proceeded from the sidewalk toward the crosswalk. 歩道, read ほどう, is the starting place, and 横断歩道 the place approached.
Facility position and the route actually used
歩道橋を使わず、横断歩道を渡りました。 says that someone crossed at the crosswalk without using the overpass. 使わず identifies the facility not used.
歩道橋の下に横断歩道があります。 places a crosswalk below the overpass. It describes both facilities in one arrangement without stating which was used.
Which route was actually crossed in 歩道橋を使わず、横断歩道を渡りました。?
Show the answer and why
The crosswalk. The sentence explicitly says that the overpass was not used.