To get to Roppongi, take the Hibiya/O-Edo subway to Roppongi H04/E23. Alternatively, walk from Tameike-Sanno G06, Roppongi-Itchome N05, or Azabu-juban N04/E22.
If you walk west under the highway long enough half an hour plus, you'll eventually end up in Shibuya; following the highway east for about as long will take you to the Diet Building and Imperial Palace. In daytime, you can also take frequent buses along Roppongi-dori to speed your way.
