As outlined in the Half-Life: Raising the Bar art book ... also appears in the Half-Life 2 beta, so it's clear that Valve has been trying to make the concept work for a long time.

In the finished Half-Life 2, there's a section where Gordon and Alyx are fighting through City 17 trying to reach Barney, and come to a destroyed bridge. You end up dropping down to the canal before a ...