This sign indicates that there is an overtaking lane coming up. You normally find these on hills and are only used if there is no oncoming traffic, logically.
You can find this between The pines and San Juan neighborhood (Taramay) in Almuñécar, for example. Eastbound traffic there has priority when using this overtaking lane.
The double lane from La Caleta nearly as far as Hotel Salobrena can only be used by traffic heading west, uphill, however, so it doesn’t apply in the case of this roadsign.
(News: Salobrena, Costa Tropical, Granada, Andalucia)