Pedestrian bridges are primarily designed to save lives.
They are often situated at locations where traffic is heavy – typically on expressways.
The also are placed at fast flowing auto traffic roads in order not to impede free traffic flow.
Despite its capacity to save lives, it still takes a great effort to get people to use them despite its capacity to save their lives.
Instead, in Lagos state, pedestrian bridges are typically converted to commercial locations, places for begging, hangouts for miscreants, etc.
See some photos of typical pedestrian bridges in Lagos.