Protection device (10) to selectively close the passage between two carriageways of a road with several carriageways such as a motorway or suchlike, comprising a horizontal protection element (14) movable between an active position, wherein it performs a function as a crash barrier, and a passive position wherein it allows vehicles to pass between said carriageways; in the passive position the protection element (14) is raised with respect to the active position, and actuation means are provided to vertically displace the protection element (14) between the active position and the passive position, while maintaining the protection element (14) substantially horizontal, in such a manner as to allow vehicles to pass below the protection element (14) when the protection element (14) is in the passive position.