`highway=` is a functional class. Although `=motorway` is quite an exception, it can be argued for following closely. Tenatively, they are distinguished by `motorway=no` + `motorroad=yes` to reflect their function and status. 
HK is complicated by Tunnel Area appearing in the middle of Expressway, as in here, and Cheung Tsing Tunnel; as well as Tsing Sha Hwy.  This doesn't play well with how `highway=` should be classified between junctions, specifically the last diverge where you can exit before entering the Tunnel Area or Expressway. 
We had short preliminary discussions on changing them https://wiki.openstreetmap.org/wiki/Talk:Hong_Kong/Transport/Road