You've missed several things. First, there's a park where you can open a valve to gain access. Inside, there's a small building blocked by roots. The game involves bleeding roots throughout the level to gain access. There's also a door leading to a bar behind the elevated road where you start. In these two areas, as well as the main one, there are roots you can knife: 5 in the main part of the city, 1 in the bar, and 2 more in the apartments where you found the zombies.