well as drivers of vehicles that may have been yellow-trapped whilst turning right a chance to clear the intersection in more safety. It also informs drivers who may be approaching the intersection at speed that a green light is imminent, so they may proceed through the junction without having to stop (or, with enough of a lead distance, even having to slow), reducing the potential annoyance (and safety risk) of braking sharply to a halt only to have the green light appear immediately after. Between green and red Traffic lights in Greece, Italy and Sweden used to show a green and yellow light together, to indicate that the light was about to change to red. All countries now officially use a single yellow light instead, as displaying both the yellow and the green lights is in the violation of the Vienna Convention on Road Signs and Signals. Traffic signal ahead Prepare to stop In some areas, a "prepare to stop" sign with two alternately flashing yellow lights is installed in locations where a high-speed road (design speed usually at least 55 mph / 90 km/h) leads up to a traffic light, where the traffic light is obscured from a distance (or both conditions), or before the first traffic signal after a long stretch of road with no signals. This is installed so that drivers can view it from a distance. This light begins blinking with enough time for the driver to see it and slow down before the intersection light turns yellow, then red. The flashing yellow light can go out immediately when the light turns green, or it may continue for several seconds after the intersection light has turned green, as it usually takes a line of cars some time to accelerate to cruising speed from a red light. These are relatively common in areas such as the United States, Canada, Western Australia, New South Wales, New Zealand and Liberia. Japan uses a variant signal with two lamps, a green one and a flashing yellow one, for the same purpose. A common way of warning that an obscured traffic light ahead is red is a red-signal-ahead sign. It is shaped like a standard yellow diamond shape sign with LEDs spelling out "Signal Ahead". Just before the traffic light goes yellow, the word "Red" will light up above Signal Ahead and they will begin to flash alternately. Strobe lights United