A box junction is a traffic control measure designed to prevent gridlock at busy road junctions. The surface of the junction is marked with a criss-cross grid of diagonal painted lines (or only two lines crossing each other in the box junction) and vehicles may not enter the area so marked unless their exit from the junction is clear (or, if turning, to await a gap in the oncoming traffic flow). In the United Kingdom and Republic of Ireland (where cars drive on the left), drivers may enter the box and wait when they want to turn right, and are only stopped from doing so by oncoming traffic, or by other vehicles waiting to turn right.
Similar yellow boxes may be painted on other areas of roadway (such as the exits from emergency vehicle depots) which must be kept free of queuing traffic.
Box junctions are currently used in the United Kingdom (where they were invented), New York City and its Tri-State region, some parts of Colorado, Cyprus, Hong Kong, Singapore, Malaysia, the Republic of Ireland, Malta, Portugal, South Africa, Taiwan, major cities in Brazil, Canada & Trinidad & Tobago
In many U.S. states, laws intending to decrease gridlocking at intersections, rail crossings, and marked crosswalks have been enacted which prohibit motorists from entering any of the three until they are certain their vehicle can clear it. No special road markings are used to indicate this rule, but some governments post "DO NOT BLOCK INTERSECTION" signs to increase awareness of the law at problematic intersections.