Today’s update for Cities: Skylines finally adds tunnels, giving you yet another option to deal with traffic issues. If you can’t go around it or go over it, now you can try going under it! The update also adds a new European tileset, although the default tileset seemed pretty European to me. The new tileset only appears on specific maps, such as the three new maps included in the update.
Read more specifics here, including a word about how to avoid conflicts with any mods you may be using.