When first visiting Route 11, you cannot leave until completing all encounters, and you cannot return until after beating the game. Climbing the stairs and approaching the second encounter will automatically start the chain of events that leaves you outside the route.
Despite its vast linear forest and pocketed elevated pond, this path hides many dangerous Nuclear Pokémon as you ascend up the gentle slope. Damage by the rampaging Pokémon has already taken its toll, as the blue river is ghostly empty.
The mission objective is to prevent the encroaching Nuclear Pokémon from breaching the defenses, and to rescue any survivors.
A nurse who can heal your Pokémon is stationed at the Legen Town entrance.
Since the gym badge for using Strength outside of battle is already in the player's possession, it is possible to skip entering this route entirely by trading over a Pokemon that already knows the move. However, if you attempt to return to Route 11 at any point after beating the 6th Gym in Venesi City, Kellyn will not appear in the transition area and give you the Strength HM, making it impossible to obtain.
Furthermore, if you choose to return after gaining the ability to Surf, you can bypass the first mandatory fight by Surfing up the river to the left.
Waiting until you are able to Fly outside of battle will allow you to constantly revisit Route 11 as long as you never trigger the final preset fights. This allow you to shiny and/or stat-hunt feral Nuclear Pokemon that otherwise become inaccessible, making it the only place players can revisit to catch Nuclear Pokemon other than Nuclear Plant Zeta, (albeit via an unintended method.)
However, if a you conquer the Pokemon Championship before completing Route 11, all Pokemon, including those in preset encounters, will no longer be Nuclear-Type. The Poke Radar will also fail to attract Nuclear-Type Pokemon even if you do not conquer any of the preset encounters.