Abstract: A removable barrier adapted to co-operate with a concrete median barrier of a roadway to form, in situ, a barrier between a vehicle travelling in one direction on the roadway and a vehicle travelling in another direction, wherein:the removable barrier is adapted for movement such that, when desirable, the vehicle travelling in the one direction can transgress the barrier and travel on the opposite side of the roadway, and further whereinthe removable barrier comprises at least one shell conforming substantially to the outer shape of the concrete median barrier and is adapted for sliding movement relative to the concrete median barrier.