From:
http://en.wikipedia.org/wiki/One-way_traffic
"An advantage of one-way streets is that drivers do not have to watch for cars coming in the opposite direction on this type of street."
The abstract 'No Entry' sign was officially adopted to standardization at the League of Nations convention in Geneva in 1931. The sign was adapted from Swiss usage. The Swiss, in turn, adopted the practice of former European states that marked their boundaries with their formal shields and, when not wishing visitors to enter, would indicate so by tying a blood-red ribbon horizontally around the shield.[citation needed] The sign is also known as C1, from its definition in the Vienna Convention on Road Signs and Signals.