trunk:
  m7 bit 6: road present
  m7 bit 7: tram present

nrt option 1:
  m4 bit 0..3, road subtype, 0xF = no road
  m4 bit 4..7, tram subtype, 0xF = no tram

nrt option 2:
  m7 bit 6: road present
  m7 bit 7: tram present
  m4 bit 0..3: road subtype, only valid if m7 bit 6 is set
  m4 bit 4..7: tram subtype, only valid if m7 bit 7 is set