Cars and pedestrians probably need different navmeshes (because different radii), and that part’s straightforward: Just define a second agent type in the Navigation options, with a different radius.
For panicking pedestrians, though, the easiest thing is probably to give the “only when you’re panicking” areas their own NavArea. Then, you blueprint a NavigationQueryFilter which rejects those areas. Then, when invoking pathfinding (MoveTo etc.), pass that navigation filter if the character is not panicking, and pass no filter if they are.