A barrier (1) has a base unit (2) which is secured to the ground and a top unit (3) which may be connected at any one of four different orientations. An electronic controller (6) is mounted in a door (5) of the base unit (2) so that is may be easily accessed. A drive mechanism (35) is mounted in a modular manner in the top unit (3). It is mounted by virtue of a beam drive shaft (21) which is mounted by bearings (22) at each end to opposed side walls (20) of the top unit (3). The beam is connected to the drive shaft (21) in a symmetrical manner so that the barrier may be used for either the right side or the left side of a laneway. There is excellent support and balance in the drive mechanism because it is connected between support bearings of the beam drive shaft (21). A manual override is provided to release a clutch if an electrical fault arises. The beam has a shear plate (27) which yields to minimise damage to either the drive mechanism or a beam arm (30) in the event of an accidental impact against the beam.