How to put a Hall Blvd overpass over the Fanno Creek Trail? An Oregonian story about a new bridge over Oregon 213 near Oregon City has a splendid graphic that gives some idea of how that could be done:
Hall Blvd, of course, doesn't have an existing overpass under which foundations for an overpass could be built while the overpass span was under construction adjacent to Hall Blvd. That might mean supports for the span under construction would have to be sufficiently strong to support use of the span once it was constructed, in the construction location. Traffic could be diverted onto the new span, and the Hall Blvd alignment could be prepared as needed for the new span to slide into place. Or...Hall Blvd could permanently be left with the bump-out change in alignment.
Most likely very expensive to do. Just how highly do people value this park...today and into the future, is a key question to high quality of crossing will be built.