Traffic-light signalling and operation
The use of traffic lights to control the movement of traffic differs regionally and internationally in certain respects. This article describes some of these non-universal features. Note that the color phase commonly known as "yellow" is often referred to, especially in official usage, as "amber"; for consistency this article uses "yellow" throughout.
- 1 Flashing beacons
- 2 Unusual traffic light phases
- 2.1 Turn indications
- 2.2 Crosswalks
- 2.3 Special signals
- 3 Warnings that light will change
- 4 Warnings of traffic light ahead
- 5 Unusual traffic light designs
- 6 Unusual uses of traffic lights
- 7 See also
- 8 References
- 9 External links
In the United States, a flashing red light is the equivalent of a stop sign. In the United States and Australia, flashing yellow does not require traffic to stop, but drivers should exercise caution since opposing traffic may enter the intersection after stopping. This may be used when there is a malfunction with the signals, or late at night when there is little traffic. A single four-way flashing light showing only one color in each direction may be used at intersections where full three-color operation is not neeeded, but stop or yield signs alone have not had acceptable safety performance. Yellow lights are displayed to the main road, to highlight the intersection and inform drivers of the need for caution. Red lights supplement stop signs on the side road approaches. All-way red flashing lights can supplement all-way stop control, but all-way yellow beacons are prohibited by US regulations.
In some parts of Canada, a flashing green (known as Advanced Green) light signals permission for a left turn before the opposing traffic is allowed to enter the intersection (i.e. oncoming traffic is facing a RED light). Similarly, a flashing green may be an Extended Green, for left turns after the opposing traffic's full green phase. The flashing light may be a "full" green, or a green left arrow, both meaning the same thing. At least one traffic light in Montreal (on the Island of Montreal, 'Right-on-Red' is not allowed), has a flashing right-turn arrow, indicating that the pedestrian crossing has a red light, so it is safe to turn right and drive across it. In other parts of the same country (e.g. Vancouver) a flashing green light conveys a very different meaning: the crossing road has stop signs with no lights of its own, and oncoming traffic also has a flashing green, not a red stop-light. This functions the same as a European "priority" sign (a yellow-and-white diamond shaped sign indicating that the current street is a Priority or "main" road, which cross streets must yield to at uncontrolled junctions, opposite to the usual arrangement in many European countries), for which there is no direct equivalent in North America.
Unusual traffic light phases
In Winnipeg, Manitoba, Canada some of the signals have a special phase where there is a red light illuminated simultaneously with a green straight arrow. The meaning of this configuration is that a motorist must not turn either left or right. An example of this is at Main Street and St. Mary Avenue. Many of these signals have been replaced in recent years with standard protected or permissive signals that are more familiar to visitors during the course of signal modernization. Also, there is a traffic light on McPhillips Street and Kingsbury Avenue that has a U-turn signal on it as the crossing road is constructed with a barrier to block a legal left turn. In the Province of Quebec, a similar function is also in use where a green straight arrow is displayed alone, usually for 5 to 9 seconds and then the full green (or right turn arrow) illuminates. This allows pedestrians to engage into the roadway, and therefore (in theory) increases safety. Soldier's Field Road in Brighton, Massachusetts at the intersection with Nonantum Road, Birmingham Parkway and North Beacon Streets always has a red ball. Green arrows appear with the red ball to allow traffic to travel in a particular direction, but the red ball always is illuminated. The same is true at the intersection of Cambridge Street and Massachusetts Avenue near Harvard Square in Cambridge, Massachusetts, and at the intersection of West 3rd Street and Mesaba Avenue in Duluth, Minnesota. This is also true in Chicago, the straight off straight on ramps from Lake Shore Drive to N Fullerton Ave display a red light and left turn arrow or a red light, never just a green arrow.
On the island of Montréal, Québec, Canada, it is forbidden to turn right while a red signal is present. At many intersections, lights will change from red to a green arrow permitting drivers to proceed straight through the intersection. After approximately 5 seconds, the green arrow is replaced with a green ball signal allowing drivers to proceed forward or turn as they wish.
In the UK, there is normally no need for red or yellow "arrow" lights, and no provision for them in law. Layouts are typically simple, either a plain three-disc all-directions signal; the same with a green left or right "filter" arrow which lights up either independently of the main green (permitting a left or right turn at an otherwise red light) or along with it (showing that any conflicting traffic has been stopped so turning traffic does not need to yield); or in multiple full sets where the horizontal positioning and lane layout (particularly where lanes are segregated by kerbs or islands) denotes which directions are stopped or free to move. However, the lack of red and yellow arrows can lead to unusual setups with non standard road layouts, such as a staggered entrance and exit to a busy retail park at one side of a mostly 2-lane (plus turning lane) road. The exit crossing has a normal set of 3-way lights (with the slight oddity that small numbers of vehicles can be momentarily "trapped" inside the junction by the inward-facing red light), as exiting traffic conflicts with both travel directions on the main road. However, the entrance crossing (from the side opposite the complex) only conflicts with oncoming traffic as it is a one-way system, and traffic on that side of the road conflicts with nothing; indeed at times it is itself mostly exit-traffic. Stopping it at the same time as the entrance flow would cause unnecessary congestion. With multi-colour arrows available, a simple all-turn red/yellow/green set could be installed, and the turning lane slightly kerbed off to make the division plain. In their absence, it is easy to mistake a red disc as an "all stop" signal... so, the straight ahead direction gets a solo permanently lit straight-ahead arrow, bolted onto the side of a red disc, yellow disc, and conflicting-turn green arrow triplet.
In New York City, some intersections have a three-lens signal without arrows with an adjoining three-lens signal (with arrows) governing exclusive left- or right-turns. Signals with circular lenses generally control all movements. At such intersections, the exclusive turn signals indicate that there are no permissive turn movements at that intersection, and that only exclusive turn movements are allowed. At those intersections, the signals with circular lenses govern movements in all directions except those in the direction which the turn signal controls. This means that if there is a left-turn signal and a circular-lens signal at an intersection, the latter signal controls right turns and straight movements, but not left turns. If the former signal is showing a red arrow, and the latter signal is showing a green ball, then motorists cannot turn left even though the latter light is showing a green ball. This is also applicable for other traffic signals around the United States.
Indication of protected turn
Throughout most of the United States a protected turn (a turn that can be made without conflicting traffic) is indicated by a steady burning 12-inch-diameter (300 mm) green arrow. This indication may be displayed in a separate traffic signal head or may be in combination with other arrows or a green ball indication on the same signal such as with the Dallas Phasing configuration described below. Modern signal standards require that a yellow "clearance" interval be displayed for not less than three seconds prior to the protected turn interval ending.
In parts of Canada (the Maritime Provinces, Quebec, Ontario, Saskatchewan and Alberta), a flashing green light has a special meaning. It is only shown in one direction, with the other three directions in a 4-way intersection having a red light. It gives the traffic in its direction the right of way in all directions; straight through, left turn or right turn. This was done because the green arrow already had a different meaning in Ontario. It meant that all traffic must turn in the direction of the arrow.
The light phase is known as "advanced green", and a sign saying "Advanced green when flashing" is usually attached to the light in question. The opposite side often has a sign attached to their lights saying "Delayed Green Wait for signal" or "Opposing Traffic Has Advanced Green." Advanced green indicates that the opposing traffic is facing a red light, and it is safe for the driver to turn left. Some of the signals had the turn phase after the oncoming green. They had signs saying "Extended green when flashing." This sequence had the hazard of yellow trap.
In Ontario, older lights with this system are slowly being phased out in favour of more universally understood left-turn arrow signals. In Alberta and Saskatchewan as a legacy, left-turn arrow signals also flash, rather than being displayed steadily as is done elsewhere.
In British Columbia, Alberta, Saskatchewan, Ontario, Quebec, New Brunswick and Nova Scotia, a flashing green arrow indicates that the opposing direction still has a red light (and any pedestrian crossing in that direction is at a "don't walk" indication), thus drivers are free to make a turn in the direction of the arrow turn. This is also used in most of the former British Commonwealth.
Protected flashing green is now used in parts of California and Ontario as part of traffic signal preemption for emergency vehicles. This does not conform to the Manual on Uniform Traffic Control Devices.
In Ireland and the UK, a right arrow may be displayed alongside a green light to indicate that oncoming traffic has been stopped and that it is safe to turn right.
In Japan, they never show a green arrow with the circular green. Instead, green arrows must be shown with the circular red. This means a signal may display green arrows pointing in all possible directions with the circular red. Another unusual sequence is that the circular red changes to circular yellow whenever some or all of the arrows end, and then changes back to red after the clearance period.
Indication of permissive turn
||This section may require cleanup to meet Wikipedia's quality standards. (July 2011)|
Flashing yellow arrow
A new configuration involving a flashing yellow arrow has been introduced and is gaining acceptance across the US. This configuration prevents "yellow trap" when properly implemented. First observed in Oregon (and elsewhere in the United States like Fullerton, California; Lawrence, Kansas; Tyler, Texas; and across North Carolina) signals displaying a flashing yellow arrow are being phased in to replace the 5-lamp protected/permissive signals that are still in widespread use.
Four models of this signal exist: The one with 4 lamps is the most common. It has a steady red arrow, a steady yellow arrow, a flashing yellow arrow, and a steady green arrow. One rare version has five lenses, with a steady red arrow, two steady yellow arrows, a flashing yellow arrow, and a steady green arrow. There are also two with 3 lamps. One signal has a steady red arrow, a steady yellow arrow, and a "dual indication" third lamp which can display either a steady green arrow or a flashing yellow arrow. The other 3-lamp signal has a steady red arrow, a steady yellow arrow and a flashing yellow arrow, with no green arrow. This last signal is used to prevent yellow trap where no green arrow left turn phase exists on the approach. This cannot be done with the circular green without causing yellow trap, because a circular green releases other movements.
The steady arrows all have their usual meanings. A flashing yellow arrow indicates a "permitted" left turn, where drivers may turn left without stopping, but opposing traffic has the right-of-way. The flashing yellow arrow must always be followed by either a steady yellow arrow or a steady green arrow.
The purpose of these signals is to allow traffic signal controllers to safely select movements as needed to maximize traffic flows, and to prevent vehicles from stacking up in left turn pockets to the point that they block through lanes of traffic. It also gives traffic engineers more flexibility with the leading and lagging protected left turn phase configurations, allowing for better progression (coordination) between signals. This saves gasoline by stopping fewer vehicles.
The flashing yellow arrow was not introduced because of a perceived lack of understanding of the circular green. Some politicians (who do not understand either yellow trap or the need for combined lead and lag signal phasing to coordinate signals on more streets) say the reason is "drivers don't understand the circular green." Note that many people don't see the difference between the flashing yellow arrow and the circular green, because both mean the same thing to the turning driver. But the important difference in meaning is in the indication given to drivers who are not making the indicated turn. The prevention of yellow trap depends on this.
To the driver turning left, the flashing yellow arrow means exactly the same as the steady green circular indication. Left turning drivers are allowed to make a left turn if there is a gap in oncoming traffic. With the flashing yellow arrow, opposing straight-ahead traffic has a circular green at the same time. With the steady green arrow, opposing traffic must have a circular red.
The flashing yellow arrow has an important safety advantage because it can be shown while the through signals on the same approach are red. The circular green cannot do this. If oncoming traffic has a lagging protected left turn (with circular green and green left turn arrow indications), the flashing yellow arrow continues to flash to drivers turning left across the oncoming traffic, even though the straight-ahead signals are red. This indication avoids the yellow trap problem.
Yellow trap occurs when the circular signal turns yellow, and then red, while oncoming traffic still has a circular green. This traps left turning cars in the intersection while oncoming traffic continues to flow. It can also fool drivers into mistakenly assuming that oncoming traffic also has a yellow light, so they turn across live traffic, causing an accident. When the opposing direction has both a circular green and a protected left turn green arrow, the flashing yellow arrow can be used for turns across oncoming traffic, but a steady green arrow can not be used.
NCHRP Report 493 and the associated Web-only document 123 shows how most subjects understood a flashing yellow arrow better than any other permissive left turn indication, and how left turn crashes were reduced after a circular green permissive left turn signal was changed to a flashing yellow arrow left turn signal.
Only four states are not using the flashing yellow arrows at all. In 2008, Maryland had planned to require that all new signals have flashing yellow arrows. But their new state MUTCD prohibits the flashing yellow arrow in favor of the flashing red arrow Maryland has used for years. The other states are Hawaii, Pennsylvania, and West Virginia. Indiana has adopted the flashing yellow arrow into its law and state manuals, and has recently completed its first installations.
The yellow trap issue was originally addressed by the configuration known as Dallas phasing (below); however, engineers felt that a flashing yellow arrow was less confusing than a louvered green ball. Dallas phasing is more problematic if the left turn signal comes out of adjustment and its indications are visible to through traffic.
Another advantage of the flashing yellow arrow face is that its operation can be changed by time of day or traffic conditions between the following modes:
- Exclusively protected (turns on green arrow, but no permissive turn)
- Protected/permissive (turns on green arrow or flashing yellow arrow)
- Exclusively permissive (flashing yellow arrow, but no green arrow)
- Prohibited (red arrow remains lit, to prevent traffic from turning across an approaching train)
Note that the flashing yellow arrow is not allowed where left turns and other traffic share the same lane.
The flashing yellow arrow is now a standard configuration for protected/permissive turn signals in the new US 2009 Manual on Uniform Traffic Control Devices, released December 16, 2009.
The circular green is still allowed as a permissive left turn display, but has new restrictions on it:
- It may not be placed over or in front of the left turn lane in new installations.
- Any yellow trap caused by a circular green must be eliminated or warned with a sign.
- If one lane is shared by left turning and straight ahead traffic, the left turn movement (protected or permissive) and the straight ahead movement must begin together and must end together at the same time in the signal cycle. In this case, a green left turn arrow may not be shown with a circular red, and a red left turn arrow may not be shown with a circular green or a straight ahead green arrow.
In the configuration commonly known as Dallas phasing that began in the Dallas/Fort Worth Metroplex, the five-light left turn signal head is used in a different manner than standard signals of this type. The left turn signal head operates independently from the signals for straight through traffic. This allows permissive turning even when straight through traffic is shown a red light. Louvers are fitted over the green and yellow balls of the left turn signal head to prevent driver confusion. The left turn signal head is also accompanied by a sign indicating its special use.
Dallas phasing gives traffic engineers more flexibility with the leading and lagging protected left turn phase configurations allowing for better coordination between signals. There are also yellow and green arrows on Dallas phasing signals, permitting exclusive protected left-turn phases as well as protected/permissive left-turn phases.
The new 2009 Manual of Uniform Traffic Control Devices prohibits this display for new installations in favor of the flashing yellow arrow left-turn signal (above).
Flashing red ball, yellow ball, or red arrow
In Michigan, a flashing red ball signal on a "protected" left turn traffic signal indicates that left turning traffic may, after a full stop, complete their turn if and only if there is a long enough break in oncoming traffic. The flashing red usually occurs when the oncoming traffic has a green signal. This function is not enabled at intersections where it may not be safe to do so (restricted view of oncoming traffic, heavy pedestrian crossings, or double-lane left turns are good examples). Michigan usually indicates signals that are dedicated to turning traffic with a sign displaying "LEFT" or "RIGHT". This sign is normally illuminated at night. More recent installations in Michigan, however, have used flashing yellow arrow signal heads, which typically have not included left-turn signal signage. All flashing red ball signals in Michigan will gradually be phased out in favor of the flashing yellow arrow. Maryland also used the flashing red ball.
Delaware and Maryland have also been known to place flashing red arrows at certain intersections, especially when no signal is needed for cross traffic. The driver is required to come to a complete stop before turning. This has been published in the 2009 MUTCD as an alternative to the flashing yellow arrow. However, this was allowed only when an engineering study determined that a "stop condition" must be imposed during the permissive left turn movement. Cupertino, California, previously used the flashing red arrow, but has changed them to flashing yellow arrows.
Washington state, particularly Seattle, used a flashing yellow ball in the left turn signal for the same purpose. The signal face contained a steady red ball, a steady yellow ball, a flashing yellow ball, and a dual color indication steady green and steady yellow arrow. It was effective in eliminating yellow trap.
No Turn Arrow
In Australia and New Zealand, an absent left or right turn arrow means traffic turning in the direction of the arrow may turn when the main light is green, provided they give way to pedestrians and other traffic.
In the state of Victoria, some intersections of this type employ a turn arrow without the red arrow. This would turn green with the main signal, before turning yellow, then off, giving priority to oncoming traffic. The nationwide standard seems to now include a red arrow that turns off. This arrow turns red simultaneously with the main light. After the cross traffic has had its turn, the arrows on opposite sides would both turn green, until one side runs out of right-turn traffic. In any case, when both sides of the intersection turn green, the corresponding arrow will turn off after a short delay, thus working similarly to the old Victorian standard. This method has the advantage of being controlled during peak-hours, where controllers would be able to prevent the arrow from turning off in extreme peak-hour traffic, but causes confusion as drivers expect a light to be on when three are present.
In New Zealand, red and yellow arrows (where fitted) always display regardless of the aspect main light. When a road is given a green light from an all-turn direction stop, a red arrow continues to display to turning traffic, holding traffic back while the pedestrian crossing on the side road is given a green signal. As soon as the pedestrian signal changes to flashing red, the red arrow extinguishes. Traffic turning may now proceed provided they give way to pedestrians and other traffic (oncoming traffic for right turns, right-turning traffic for left turns). This method is becoming common in many states of Australia. When an intersection is given a protected turn prior to the pedestrian crossing on the side road given the green signal, the lights change to yellow and red, and then the red arrow disappears as soon as the pedestrian crossing is given a flashing red signal.
The 5-light protected permissive signals in the US are the same.
Flashing green light
In British Columbia, a flashing green globe signal is used at a pedestrian crossing or intersection, at which pedestrians have the ability to stop traffic to allow a safe crossing. They may also be used at a drawbridge. The flashing green indicates that the signal is not currently in use. After the pedestrian pushes the button to trigger the signal, the light becomes a steady green until the sequence of yellow, then red (at which time the pedestrian crossing gives a walk signal) as in a conventional set of traffic lights, then returns to flashing green until another crossing is requested.
In Ontario, a flashing green indicates a protected "advance green" during which traffic may proceed straight through the intersection or turn left or right, as allowed at any particular junction and from the proper lane; opposing and cross traffic have a red signal. Ontario's flashing green signals have largely been replaced by green arrows after provincial standards were rewritten in 1999.
In Massachusetts, specifically in Cambridge and Somerville, the main street will have a flashing green signal, while cross streets have a signal that have a red on top, yellow in the middle and flashing red in the bottom position. When a pedestrian activates the signal, the cross street changes from flashing red in the lowest position to yellow to red (topmost position).
In several European countries and Mexico, a flashing green light is used in crosswalks to indicate that signal is going to change from green to red soon. Therefore, flashing green has roughly the same meaning to pedestrians as ordinary yellow signal has for motorists. Slow-moving pedestrians are warned about oncoming signal change and have opportunity to wait for next signal cycle. Motorists are more likely to notice flashing signal. Drivers of vehicles about to cross pedestrian crossings should be more aware of incoming pedestrians.
Current users of flashing green signal are Austria, Belgium, Estonia, Finland, Great Britain, Hungary, Netherlands, Norway, Mexico, Poland, Spain and Sweden. France, Portugal and Switzerland make limited use of flashing green.
Flashing red lights
In Australia, New Zealand, Canada and most of the United States, a flashing red pedestrian signal is used at between green and steady red; it means "complete crossing but do not start to cross". This has a similar meaning to European flashing green, but means that if a pedestrian glances at it, they will not enter an intersection without enough time to leave. In the United States and in parts of Canada and New Zealand (e.g., Auckland CBD) pedestrian signals which count down the number of seconds (see Timers below) until cross traffic has the right of way are becoming popular at heavily used pedestrian crossings such as in urban shopping districts
In New Zealand, paired red/red traffic lights are often installed outside Fire and Ambulance Stations on major roads, which when activated by the station, flash alternatively (so that at any time one red light is always showing), the purpose being to cause to traffic to stop for a set amount of time to allow emergency vehicles to exit their station safely
Red and yellow light
In Massachusetts only, simultaneous red and yellow lights in all directions allow a pedestrian to cross diagonally. This replaces the extra "WALK/DONT WALK" signal (which MUTCD forbids.)). As of 2011, this practice was phased out; it no longer appears in the Commonwealth's driver's manual.
In a number of countries, the traffic signal will briefly show red and yellow together to indicate the imminent transition to green; see the Change From Red To Green section on this page.
Traffic lights for pedestrians are usually different; see pedestrian crossing. Traffic lights at level railroad crossings are again different. Both of these are to avoid confusion as to whom the signal applies.
Transit priority signal
In Oregon, Ontario, Quebec, British Columbia, Nova Scotia, Manitoba and Alberta, traffic signals may also have an extra white rectangular light mounted above the red light. This phase indicates that a public transit vehicle may proceed through an intersection in any direction while all other traffic faces a red light.
In some areas such as Boston, Massachusetts, a trolley may have its own traffic signals, indicating that it is okay for it to cross an intersection. These signals are all white, and the top section (stop) is a horizontal bar, the middle (caution) is an upright triangle, and the bottom (go) is a vertical bar.
In Australia and New Zealand, buses and trams may have a white "B" and "T" light respectively to indicate they may proceed through the intersection in any direction. A white arrow indicates that they only may proceed in the arrow's direction, common for trams to indicate that they may proceed and the points are set for proceeding that direction. Transit signals may be accompanied by red and yellow B/T signals indicating to buses and trams stop and caution respectively.
In many parts of Western Europe transit signals (for trams, and in some cases buses as well) employ traffic signals that are phased similarly to main traffic signals but replace the green light with a vertical white bar, the red light with a horizontal white bar and the yellow with a white dot or diamond. This is intended to avoid confusion between transit signals and main traffic signals at intersections where both sets are visible.
Emergency priority signal
At many intersections in the United States, intersections use traffic signal preemption to give priority to emergency vehicles. These preemption applications often include an illuminated "notifier" signal. A notifier is a secondary lighting device usually mounted independently of the traffic signal, such as a standard or strobing light bulb in an omnidirectional enclosure or spotlights aimed at oncoming traffic lanes. The colors of these secondary lighting devices vary regionally depending upon the operational policies of the local traffic management and emergency service agencies.
Reverse side red light indicator
Some jurisdictions use special small blue lights on the reverse of signal heads to indicate a red light lit on that head. They are used to communicate the presence of a red signal to police so they can view the situation without having to traverse the intersection. Other jurisdictions simply drill a small hole in the red signal visor to allow police to see the status of the signal from a wide angle (but not directly opposite the signal).
Warnings that light will change
Change from green to yellow
In Austria, Cambodia, Estonia, Latvia, Lithuania, most of Israel, parts of Mexico, Turkey, Russia, and in certain other parts of Europe, the green lights will start flashing at the end of the Go or Turn phase to indicate that the yellow (Caution phase) lights are about to be engaged. This is useful in fast paced roads to allow for longer slowing down time, and for pedestrians crossing broad streets. Some traffic lights in Pennsylvania illuminate the yellow light a few seconds before the green light turns off, to give this same warning.
Note that the 2009 Manual of Uniform Traffic Control Devices prohibits any display that gives warning of an upcoming signal change, unless that display is placed well upstream of the signal (See "Warnings of traffic light ahead" below), so traffic at the stop line can not see it.
Change from yellow to red
At some intersections in Quebec, Canada, the yellow and red lights will appear together to indicate that the light is about to change to red. This mitigates the fact that at most Quebec intersections, there is no delay between the time that the lights in one direction turn red and the lights in the perpendicular direction turn green. However, this is considered redundant in other places, as the yellow light itself indicates that the light is about to change to red.
Change from red to green
In most European countries (including Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Croatia, the Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Iceland, Latvia, Lithuania, Macedonia, Malta, Moldova, Montenegro, Norway, Poland, Russia, Serbia, Slovakia, Slovenia, Sweden, Switzerland, Ukraine, and the United Kingdom), as well as in Argentina, Colombia, Hong Kong, Indonesia, Israel, Pakistan, and Paraguay the red and yellow lights are displayed together for one, two, or three seconds at the end of the red cycle to indicate that the light is about to change to green. This phase aids the drivers of vehicles with manual gearboxes, giving them time to change into first gear during the short phase, as 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.
Warnings of traffic light ahead
Flashing yellow lights
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, and New Zealand. Japan uses a variant signal with two lamps, a green one and a flashing yellow one, for the same purpose.
Red signal ahead
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.
Strobed red lights
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.
Unusual traffic light designs
Double red lights
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:
- green – an ordinary "ball" shape,
- yellow – a diamond shape, and
- red – a square (somewhat larger than the ball shape).
In Quebec, most horizontal traffic lights have a red signal on both sides of the fixture (left and right). They are also now replacing the shaped traffic lights for color-blind people with regular round signals.
Some signals have two red lights, one on each end.
In some Texas urban areas including Houston and Dallas, the use of a double red light is different. It is typically used on left turn signals. For horizontally mounted signals, typically hung or mounted over the lanes, it is configured with two red balls or arrows, one yellow arrow or ball, and a green arrow (from left to right). For vertically mounted signals, the two red balls or arrows are on the top, then a yellow arrow or ball, and a green arrow. It is usually accompanied by signs saying "left turn signal" or "left on arrow only". Signals for traffic going straight use standard signals, usually mounted horizontally over the road. The use of two red lights on the left turn signal allows for redundancy in case one of the red lights burns out, while saving money by requiring only one signal for left turns per direction that needs one. It also prevents the yellow trap that would occur at night if a single red signal burned out, and left-turning vehicles obeyed the circular signals instead.
One type of installation in Texas uses a double red light instead of a single red light to make the red light more pronounced and visible from a distance. In this installation, it is the first traffic light on a rural highway for miles, and traffic approaches at highway speed (65 mph). The double red light makes the red phase of the light visible at a greater distance than the yellow and green on the same signal. This installation is also used on rural highways in California, always in a vertical configuration.
The double-red light also makes an appearance in North Carolina in left-hand turn lanes. It serves the same purpose as the vertical Texan configuration, and mainly appears on boulevards with multiple lanes of traffic where a left turn may become catastrophic due to visibility. Double red arrows are used for left turn installations on some county-managed roads in Henrico County, Virginia and Baltimore County, Maryland.
The double-red ball aspect is used in Saskatchewan and Alberta, Canada, to indicate a protected-prohibited left turn signal. A sign with the universal no left turn symbol and a depiction of the double red light is mounted near the signal to indicate that no left turns are permitted on a double-red light. Intersections with this configuration are quite common in Saskatoon, Calgary, and Edmonton.
Traffic lights in Tianjin, China
- One system is where there is a horizontal bar in a specific colour, with the colour changing and the bar shrinking. The shrinking bar indicates the time remaining in that colour. The colour itself is either red (stop), yellow or green (go). A blinking green one-third-full bar means "reduce speed now", and a blinking yellow full-bar indicates "proceed with caution".
When lights of this system turn from green to red, the diminishing green bar will flash once two-thirds (note: not the full bar) of the green bar is "eaten up", with the remaining third intact. A full, uninterrupted yellow bar will appear for a few seconds before, after a short blink, lights turn red. Immediately after the full red bar appears, a tiny (almost unnoticeable) split/division appears to signify the bit that will not be "eaten up". This corresponds to the usual position of a red light (leftmost, or rightmost if at the other end of the road and at the other side of the pavement; or the upper third). When two-thirds of the red bit is "eaten up", the red light extinguishes, only to be replaced nearly immediately with a full chunk of green (again with the minute division). The process then repeats itself.
- Another system, which is also common in the other cities in China, is where there is a set of three lights as traffic lights. Every light is an arrow pointing in a different direction and every arrow has a colour of its own, to show whether traffic flow is permitted or prohibited in that direction.
The major disadvantage of this system of traffic light is that it is unfamiliar to those who are used to seeing specific colours of the traffic lights at the various ends of a normal traffic light itself (e.g. green rightmost, red leftmost, etc.) as well is being problematic for the color blind (although by Chinese traffic laws, people who are color blind are not permitted to drive). It does, however, conserve space. The other disadvantage of it is it does not have indication of when a turn can be made without yielding and when the turn can made only after yielding to the oncoming traffic. Although by Chinese traffic laws, turning is always supposed to be made after yielding to oncoming traffic.
Elsewhere in China, a blinking green light means "reduce speed now", attempting to stop cars from passing (if that car can still safely stop in time) and is nearly universal in appearance. Some cities or parts of cities show the number of seconds remaining in a specific traffic light colour (a so-called "countdown meter").
Another type of signal that can be found in China is the Unilight signal that displays all three colours in one signal section.
Unusual uses of traffic lights
A ramp meter or metering light is a device, usually a basic traffic light or a two-phase (red and green, no yellow) light, that regulates the flow of traffic entering freeways according to current traffic conditions. They are intended to reduce congestion on the freeway in two ways. One is to ensure that the total flow entering the freeway does not exceed the capacity at a downstream bottleneck. A second is to break up platoons of vehicles entering freeways, ensuring that traffic can merge more easily. Some metered ramps have bypass lanes for high-occupancy vehicles, allowing car-poolers and buses to skip the queue and get directly on the highway. Meters often only operate in rush hour periods.
On some large toll bridges, such as the San Francisco – Oakland Bay Bridge, red/green traffic lights, similar to ramp meters, are used to stagger traffic leading into the bridge. In the Bay Bridge's case, approximately 25 lanes of toll booth traffic are reduced to five lanes of bridge traffic in about 1/2 mile. To accomplish this, an overhead red/green traffic light is visible above each lane, several hundred feet beyond the toll plaza. Green is illuminated for 2 seconds, signalling the first driver in that lane to begin acceleration. Then the signal jumps to red for eight seconds. Using this method, there are always five lanes with a "green" signal, staggered throughout the 25 lanes of traffic.
Traffic lights are sometimes accompanied by timers that indicate how much longer a certain phase will last. This is especially common for pedestrian crossing lights in high-traffic areas. Timers have been extensively used in India, Indonesia, China, and Thailand for both pedestrian and vehicular traffic.
In Canada and the United States, most pedestrian signals now have countdown timers in the flashing hand symbol/"Don't Walk" phase. All new installations of pedestrian signals in the United States must include a countdown timer, unless the countdown timer is less than seven seconds long, per the 2009 MUTCD. In New York City, however, this is not the case, as only streets that are wide enough will get countdown timers, regardless of the length of the countdown. Countdown pedestrian signals are also used in London, United Kingdom.
In some cities (such as Newberg, Oregon; Kiev, Ukraine; or Kraków, Poland) there are signs displaying how fast one has to drive in order to reach the next intersection at the exact time when the light turns green, thus allowing the driver to ease into a green wave.
Other places where there may be traffic lights (normal or special ones):
- flashing signals used in conjunction with warning signs (such as dangerous curves, speed limit reductions, school zones, signal ahead, low clearance, flooding, icing, or fog) and regulatory controls (such as Stop and Yield signs).
- on waterways with signs to implement special reduced speed "no wake" zones for watercraft.
- at public boat ramps to warn drivers before accidentally driving into the water.
- at the landing-stage of a ferry and aboard the ferry.
- at the entrance and exit of a parking place or garage.
- at the entrance and exit of some car washes, to indicate when the engine should be in gear and whether the brakes may be operated at a given time.
- at drive through lanes such as those at banks.
- at an international Port of Entry inspection station.
- at highway inspection and/or weigh stations.
- before a drawbridge.
- before a narrowing of the road.
- at a fire station or medical emergency entrance.
- at a tunnel entrance.
- in some HOV lanes.
- to allow cattle to cross – as on the A470 in Wales or A65 in North Yorkshire, England.
- at road construction sites to regulate temporary two-way traffic over a single open lane.
- at airports to regulate aircraft taxiways.
- adjacent to some airports where vehicular traffic on highways (crossing just past the end of a runway) must stop or yield during aircraft takeoffs and landings.
- at the entrance to water slides where they are used as a safety feature to prevent people going down slides too soon after each other.
- on automobile racing circuits to advise race drivers whether they can race or must slow or stop
- at the end of a dead-end road to warn that the road ends, usually installed in locations where many accidents have occurred.
- Government of Alberta, Chapter 2, page 49
- McCaffrey, Gordon (1964-9). Electronic Genie Speeds Traffic Through 1001 Lights. Popular Science.
- Groh-Gordy, Michelle (2006-10-01). "Red, stop. Green, go. Flashing red, stop. But flashing green?". Inland Valley Daily Bulletin. Retrieved 2008-04-16.[dead link]
- "www.npa.go.jp/annai/license_renewal/english.pdf" (PDF). Retrieved 2013-09-17.
- "Fixing the Yellow Trap". Projects.kittelson.com. Retrieved 2010-10-18.
- "Top News". Synchronex-usa.com. Retrieved 2010-10-18.
- "NCHRP Report 493". Transportation Research Board. Retrieved 2011-06-28.
- "Web Only Document 123". Transportation Research Board. Retrieved 2011-06-14.
- "Maryland Manual on Uniform Traffic Control Devices - 2011 Edition". Maryland State Highway Administration. Retrieved 3 April 2012.
- "MUTCD Section 4D.13".
- "MUTCD Section 4D.05".
- "MUTCD Section 4D.17".
- "MUTCD Section 4D.19".
- MUTCD 2009 Edition, Chapter 4D, sections 4D.18 and 4D.20 prohibit the use of circular green in a separate left-turn signal face. However, existing installations can remain for the remainder of their useful life.
- "Flashing Yellow Arrow Left-Turn Signal". Michigan Department of Transportation. Retrieved 2008-01-03.
- MUTCD 2009 Edition, Chapter 4D Sections 4D.18 and 4D.20
- ICBC, Chapter 3, page 9
- "BC Ministry of Transportation and Infrastructure FAQ". Th.gov.bc.ca. Retrieved 2013-09-17.
- "Ontario: Advance green". Mto.gov.on.ca. Retrieved 2013-09-17.
- "Look Out, the Traffic Lights are Changing". Finance.groups.yahoo.com. Retrieved 2013-09-17.
- MUTCD 2009 Edition: Chapter 4D, item D of paragraph 3 states specifically, "A flashing green signal indication has no meaning and shall not be used."
- Page 82, Chapter 4: Rules of the Road, Commonwealth of Massachusetts Driver's Manual, 2008 edition (Massachusetts Registry of Motor Vehicles).
- "MUTCD 2009 Edition, Chapter 4D". Mutcd.fhwa.dot.gov. Retrieved 2013-09-17.
- Manual on Uniform Traffic Control Devices, Section 10D.07
- "Giving way at controlled intersections - New Zealand Road Code". New Zealand Transport Agency. Retrieved 13 June 2011.
- Usenet Message-ID 3E1475C9.738C8F95@alum.wpi.edu (January 2, 2003)
- Griggs, Gregory W (2006-09-06). "Simi Valley Adds Blue Traffic Lights". Los Angeles Times (Ventura County edition). Retrieved 2008-04-16.
- This position was formalized on July 2, 2003 with an Official Interpretation of the MUTCD, number 4-263(I), "Strobes Florida DOT".
- Manual on Uniform Traffic Control Devices, Frequently Asked Questions - Part 4 Highway Traffic Signals, Question 21
- MUTCD 2009 Edition, Chapter 4D Section 4D.06, Paragraph 3 contains a new prohibition against strobes in traffic control signal
- "Pedestrian Countdown Signal Locations" (PDF). Retrieved 2013-09-17.
- Lydall, Ross (21 June 2010). "You have six seconds to cross: digital countdown installed for pedestrians". London Evening Standard. Retrieved 24 July 2011.
- Left-turn Signal Display Animation Page from Evaluation of Traffic Signal Displays for Protected/Permitted Left-Turn Control (National Cooperative Highway Research Program report 3-54)