Something you can physically cross is passable. A passable trail through the woods isn't blocked by obstacles like fallen trees or flooded streams.
After a hurricane, the roads you normally travel may not be passable because of dangling power lines or tree branches. This definition dates from the fifteenth century, literally meaning "able to be passed" or "able to be crossed." A later meaning of passable is "acceptable." You can use this word to describe things that aren't wonderful, but are good enough: "It's not the moistest cake I've ever tasted, but it's passable."