Route 18 is a climb up to Mt. Icepick. The Route is a counterpart to Route 15. It is initially blocked off by a rockslide until the 4th Pokémon Gym is defeated. There are three different sections of Route 18, ranging from the outside climb, to the snowy section near Icepick City. You can meet Suicune on this Route, too.
You can find Suicune here for the first time. When you return after the Pokémon League, you can see Suicune again. After you see it again, it will roam the Kamla Region.
Edit