Applied to a system of directing road traffic round a roundabout or through a system of one-way streets to avoid the need for one line of traffic…

Of a street, road system, etc.: that permits traffic to travel in only one direction. Of traffic: permitted to travel, or travelling, in only one…

spec. Of (esp. rush-hour) road traffic, its flow, or a road carrying it: that uses the same lane(s) for travel in opposite directions, depending on…

Of road traffic: flow or movement alongside and in a direction contrary to an established or usual flow, as during road-works. Chiefly attributive.