<P> 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 . </P> <P> 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 . </P> <P> In some parts of the United States, a few traffic lights have slowly flashing white strobe lights superimposed on the center of the red light, which are activated when the red light itself is illuminated . These are common on highways with few traffic signals, in high - traffic, and / or high - speed areas (where drivers running red lights are a major problem), in a place where a regular traveler wouldn't expect a signal (such as a newly erected signal or one put up for construction) in other situations where extra work may be needed to draw attention to the status of the light (such as in an area where many other red lights approximate the brightness, placement and color of a red traffic signal), or the strobe may also be a flash from a camera located within the traffic signal itself (there has been much dispute as to whether this is legal or not). These are also used in areas prone to fog, as the strobing white light may be visible from a distance while the standard red light is not . A newer variant uses a flashing white LED ring located on the outer edge of the red indication as opposed to in the center of the red . Typically one strobe equipped signal is mounted as a supplement between two normal signal heads . It is worthy of note that such strobe installations have been prohibited by the FHWA since 1990; however, individual states have been slow to conform . The current MUTCD (2009 edition) contains an explicit prohibition against their use; therefore, it is still FHWA's position that strobe lights are not allowed in traffic signals and no further experimentations with these types of strobe lights in traffic signals will be approved . </P> <P> The Canadian provinces of Quebec, New Brunswick and Prince Edward Island generally use horizontal traffic lights with red to the left and green to the right . These signals also use specific shapes for each color, which aids color - blind people in distinguishing signal aspects: </P>

When do flashing red and yellow lights control some intersections