A side barrier for a truck or trailer, the side barrier comprising at least one barrier member and at least two support arms between which the at least one barrier member hingedly extends, the at least two support arms being provided with hinges one for connection to the at least one barrier member and the other for connecting the side barrier to the vehicle in a position lying parallel to the longitudinal length of the vehicle, wherein all the said hinges are arranged with respectively parallel axes. Usually a second barrier member is present via which the other hinges connect the support arms to the truck or trailer. The side barrier is for lowering the window below pre-existing side barriers for preventing or reducing accidents involving vulnerable people such as pedestrians or cyclists and vehicles.