The middle lane is not shared, it will only be used by one direction at a time and a middle barrier is usually separating these two. Wikipedia has an article ex

The middle lane is not shared, it will only be used by one direction at a time and a middle barrier is usually separating these two. Wikipedia has an article explaining it further: http://en.wikipedia.org/wiki/2%2B1_road http://en.wikipedia.org/wiki/2%2B1_road