A roundabout is a type of circular intersection or junction in which road traffic flows almost continuously in one direction around a central island. The modern form was standardised in the United Kingdom.[citation needed] So-called "modern" roundabouts require entering traffic to give way to traffic already in the circle and optimally observe various design rules to increase safety. Compared to stop signs, traffic signals, and earlier forms of roundabouts, modern roundabouts reduce the likelihood and severity of collisions by reducing traffic speeds and minimizing T-bone and head-on collisions. Variations on the basic concept include integration with tram and/or train lines, two-way flow, higher speeds and many others