No, you cannot.
You can only get to route 15 once you beat the game. You have to cross the Marvelous bridge which is inaccessible until after you beat the E4.
>In Black 2 and White 2, the bridge is inaccessible until the Elite Four is defeated. Afterwards, if the player brings a Lunar Wing to the center of the bridge, a Cresselia will appear.
Source (that and I remember form playing it)
Hope I helped. :]