"I knew I shoulda taken that left turn at Albuquerque!" -- Bugs Bunny, c/o Warner Bros.

More than three decades ago, engineers implementing traffic signals learned that they could significantly improve overall traffic flow at congested intersections by giving left-turn traffic a protected phase -- in other words, an opportunity to turn left without having to yield to other traffic, particularly oncoming vehicles. The protected phase is also valuable when, due to a hill or curve, drivers wanting to turn left cannot see oncoming vehicles in time to yield.

A steady green left-turn arrow (independent of any ordinary green dot) became the indication for this privilege, and it has become standard in the United States. Two examples of left-turn signals are shown below. The green arrow is usually followed by a steady yellow arrow when the protected phase is about to end. In most areas, a green dot without any arrow signifies the permissive phase -- a left turn is not protected, but is nonetheless permitted if the driver yields to conflicting traffic.

Since the number of cars on the road is ever increasing, engineers continue to seek refinements of the signals used with left turns in order to expedite traffic and increase safety. At some intersections, an efficient traffic light plan would include a phase where drivers seeking to turn left be given a permissive turn, while drivers in adjoining lanes intending to travel straight must stop. Under traditional signaling practice, there is no good way to indicate this to the motorists.

In one signaling scheme that illustrates this problem, drivers approaching this intersection would see a green dot and red dot simultaneously -- green for the permissive left turn, and red for all other movements. This would likely lead to confusion, and some drivers might think the traffic lights were broken. Of course, it is possible to mount LEFT TURN SIGNAL and THRU TRAFFIC SIGNAL signs next to their respective lights, but drivers may not at night see the signs in time to react. Moreover, centering the lights over their respective lanes, or installing louvers that limit the spread of light, will give drivers minimal help if the road curves as it approaches the intersection.

As a result, there has been discussion of devising some special signal, instead of an ordinary steady green dot, to be used solely to indicate the permissive left turn.

The Kittelson web site cites a number of experiments with various methods, and comes out strongly in favor of using the flashing yellow arrow for the permissive turn. We agree with Kittelson in part: at heavily-traveled intersections some special indicator for the permissive turn is a good idea. However, we disagree with Kittelson's recommendation of the flashing yellow arrow.

The obligation to yield is a totally new message for the yellow light, one not especially similar to present denotations. In one current meaning, a flashing yellow dot, whether it appears as part of an intersection control signal or as part of a roadside warning sign, indicates merely that a driver needs to use caution. In another current meaning, a steady yellow dot or arrow at an intersection implies that some privilege will end in a few seconds. Examples of this second sense:

• A steady yellow dot is generally followed by a red dot, which means that drivers (in the absence of special signals) lose the privilege of proceeding straight or turning left, as well as the privilege of turning right without stopping.
• A steady yellow left-turn arrow is sometimes followed by no arrow at all, which means that drivers lose the privilege of turning left with protection, although they may retain left-turn permission according to the other signals.
• A steady yellow left-turn arrow is othertimes followed by a red left-turn arrow, in which case drivers lose any privilege of turning left until the signal turns green again.

Another matter requires attention. For three-quarters of a century, traffic lights have communicated the beginning of a clearance interval by going from green to yellow. Further, many lights have in the past few decades been programmed with a second clearance interval, signaled by going from yellow to red. Either way, there is a change of color. On the other hand, Kittelson's proposal calls for changing from flashing yellow to steady yellow to indicate the beginning of the permissive turn clearance interval; this lack of color change is inconsistent with traditional signaling practice. In mitigation, Kittelson does recommend a signal with two yellow arrow sections, one strictly for flashing service and one strictly for steady; the shift from one signal section to another helps emphasize the beginning of clearance. However, we believe that this message can be delivered more simply and less subtly, as follows.

We suggest the flashing green arrow to signify the permissive left turn. When left-turn permission is about to end, a flashing yellow arrow appears for a few seconds to announce the clearance interval, followed by a steady red arrow. Traffic engineers may program a second clearance interval under this red light as desired.

This should have a clearer meaning to drivers. If nothing else, the flashing green light has virtually no precedent in the United States, so drivers will not have to "unlearn" any ideas. This idea also extends readily to right turns, when they require precise control.

Throughout this document, the acronym TFGAP will denote This Flashing Green Arrow Proposal. We can more specifically write TFGAP-I, where 'I' stands for intersection. Other branches of TFGAP are summarized in TFGAP-A.

Adoption of TFGAP would require a number of changes to the Manual on Uniform Traffic Control Devices (MUTCD) of the United States Department of Transportation Federal Highway Administration.

Drivers turning left ignore any signal for thru traffic. Rather, they rely on a dedicated left-turn signal with three sections: a red arrow, a yellow arrow and a green arrow. At any time, one and only one of the sections is lit, and it is either steady or flashing.

The rate of flash would presumably be between 50 and 60 cycles per minute -- the norm for a flashing signal (MUTCD 4D.11). However, a higher rate might be considered the following reason. Left-turn permission ends with a yellow clearance phase of three to six seconds (MUTCD 4D.10), and there need to be enough flashes during this time to clearly communicate to drivers that the light is indeed flashing and not merely flickering due to malfunction. A higher frequency clarifies this matter.

	Description	Followed by	Comments
	Steady Green Arrow (SGA) indicates a protected left turn; conflicting vehicles and pedestrians are stopped.	SYA to indicate that the protected left turn phase is coming to an end.	Although a driver turning under a steady arrow should not have to yield to any conflicting traffic, the wise motorist will always be watchful for errant vehicles or pedestrians.
	Steady Yellow Arrow (SYA) clears traffic at the end of the SGA phase, prior to any conflicting phase.	SRA to end left turns entirely. FGA to start a permissive left-turn phase. FRA to start a permissive left-turn-after-stop phase.	This yellow clearance phase lasts only a few seconds.
	Steady Red Arrow (SRA) indicates that left-turn traffic must stop and wait for the light to change.	SGA to start a protected left-turn phase. FGA to start a permissive left-turn phase. FRA to start a permissive left-turn-after-stop phase.
	Flashing Green Arrow (FGA) indicates a permissive left turn, where drivers must yield to conflicting vehicles and pedestrians.	SGA to upgrade permission to protection. FYA to indicate that the permissive left turn phase is coming to an end.	The sequence SGA-SYA-FGA is one of the few times that a green light can follow a yellow one.
	Flashing Yellow Arrow (FYA) clears traffic at the end of the FGA phase, prior to any conflicting phase.	SRA to end left turns entirely. FRA to start a permissive left-turn-after-stop phase.	This yellow clearance phase also lasts only a few seconds, but long enough for the flashing pattern to become evident.
	Flashing Red Arrow (FRA) indicates a permissive left turn, where drivers must first stop and then yield to conflicting vehicles and pedestrians.	SGA to start a protected left-turn phase. SRA to end left turns entirely. FGA to start a permissive left-turn phase.	In all likelihood, the left-turn FRA will never be implemented widely.

Until motorists become familiar with this signaling scheme, an educational sign can be used with the left-turn signal. In the typical case where the FRA is not employed, one of the signs pictured below can be mounted next to the signal. The message is in fact short enough that a bilingual sign becomes feasible.

Highway departments that want to increase target value of the red aspect of this signal can try the arrangement below left. Using an oversize section for the red arrow (below right) is not currently an option, because MUTCD 4D.15 specifies only two nominal diameters for intersection signals, 200 mm and 300 mm, and further requires that arrows of any color be housed in the larger of these. Still, there may eventually be a way: over the decades traffic signs and signals have become larger, and if the trend continues regulators may someday allow the use at intersections of a 450 mm light, which size is already a standard for lane control signals (MUTCD 4J.3).

If planners decide that placing the steady and flashing aspects of a color in separate sections helps drivers notice the difference, the signal can be arranged like this:

The FRA pointing left has already been installed in a few specialized situations. One example is a tee intersection where the main road is two-way, and the stem road is one-way, carrying traffic away from the main road. Receiving the FRA are motorists on the main road who want to turn left onto the stem road, and they need yield only to oncoming traffic. Such a signal becomes valuable if its controller is connected to traffic-detection equipment: when a long queue of cars has accumulated in the left-turn lane, the controller can change the FRA to a SGA (with red dot for oncoming traffic) to release the queue.

5A. An intersection with FGAs for traffic turning left may well have traditional dot signals for vehicles making some other move, so for the discussions below we introduce the obvious abbreviations SGD for steady green dot, SYD for steady yellow dot and SRD for steady yellow dot. We will also need to mention the flashing red dot (FRD) and flashing yellow dot (FYD). In some branches of TFGAP the flashing green dot (FGD) comes into play.

Besides left turns, arrows can as well be used for thru movements and right turns. When it is necessary to be specific we write SGA-L for a left-turn SGA, SGA-R for a right-turn SGA, and SGA-S to denote an SGA for vehicles headed straight. Other symbols are similarly affixed.

5B. We write plain crossing to denote what is by far the most common kind of intersection. It might have three, four or more legs, and any of them might be one way; usually the intersection is controlled by signs or signals. Examples of junctions that are not plain crossings are circular intersections and interchanges, although the ordinary diamond interchange happens to include two plain crossings.

5C. The MUTCD does not have good names for the two major modes of traffic light operation, so we assign some:

• Dynamic service is the most common manner in which traffic signals operate. Vehicles approaching from any direction will see a sequence of green, yellow and red lights. In the simplest implementations, the lights are controlled by an ordinary timer; sophisticated systems react to the amount of traffic proceeding in each direction by shortening or lengthening phases, or even skipping them entirely, to reduce overall delays. This page deals primarily with dynamic service.
• Static service (section 8 of this page; also TFGAP-S) is the alternative to dynamic. In one version, all drivers see flashing red lights; in the other, vehicles on the main road get flashing yellow while everyone else gets flashing red. In either of the all-red or yellow-and-red modes, a steady green arrow can be used for certain movements.

6A. The Sequence SGA-SYA-FGA. The table above specifies a yellow clearance phase when downgrading left turns from protection to permission, but this is not the only possibility. Suppose that at some TFGAP intersection under particular conditions of traffic, the timing of the lights is thus:

• SGA, 30 seconds
• SYA, 5 seconds clearance phase
• FGA, 60 seconds

• SGA, 30 seconds
• FGA, 65 seconds, of which the first 5 seconds is clearance phase

• SGA, 30 seconds
• Both SYA and FGA, 5 seconds clearance phase
• FGA, 60 seconds

An inefficiency of the first schedule arises from the fact that the SYA does not indicate what signal might follow; it could be any of SRA, FRA or FGA. A motorist might stop on the SYA only to discover that it is followed by an FGA. Had he known that the FGA was slated to appear, he perhaps could have turned safely while yielding but not stopping.

The second schedule incurs a safety risk when a driver approaches the SGA at a prudent speed and expects no requirement to yield; then when he is only a few meters from the intersection the light changes abruptly to an FGA. Although clearance phases routinely occur under yellow and red lights, it is most extraordinary to have one under green (in this case the FGA), and the driver may not realize that there is indeed a clearance phase for his benefit. Hence he brakes to a panic stop because he has not evaluated the other traffic and is not prepared to yield as is ostensibly required by the FGA. Naturally, panic stops such as this promote rear-end collisions.

The third schedule addresses the matters of inefficiency and driver surprise, but at the price of complexity. A motorist who does understand this sequence knows that when he sees the SYA and FGA together he first is getting a protected clearance phase, and subsequently will receive a permissive phase. On the other hand, a driver who does not understand this sequence will likely be completely baffled by a signal where for a few seconds two arrows pointing the same direction are illuminated, yet they are of different colors, and one is flashing while the other is steady. We do not know of any succinct educational sign that can explain this to the unfamiliar motorist. Inevitably, confused drivers cause crashes.

In our judgement, the efficiency problem in the first schedule is more acceptable than the surprise problem in the second or the education problem in the third.

6B. The Sequence FRA-SRA. The chart above calls for a yellow clearance interval after any green light that is to be followed by a more restrictive signal. Another sequence that increases in restrictivity is FRA to SRA, but in this red instance the chart does not mention any clearance phase. There is a reason for this difference.

Under a green signal, drivers often approach an intersection with little expectation of stopping. Once they are very near the intersection, if the signal suddenly changes to something more restrictive they may have no hope of braking sufficiently in time. Here lies the need for an explicit yellow clearance interval: it informs drivers that if stopping is impossible, they are free to proceed.

Under the FRA by contrast, cars must stop before they enter the intersection, even if the roadway is conspicuously vacant. If the FRA changes to an SRA, the driver who is stopped at the light should see the difference and wait for the light to change again. If he instead is busy looking for an opening in traffic he might miss the signal change; yet his outdated knowledge of the signal poses minimal safety risk because if he continues to believe that he has an FRA, he will remain stopped until traffic is clear.

Engineers who are concerned about this reduction of privilege without a clearance phase can nonetheless use TFGAP to good effect. Among their options:

• Never use the FRA.
• Use the FRA, but not in the sequence FRA-SRA.
• In the FRA-SRA sequence, set the change to happen a few seconds before otherwise necessary, to effect an implicit red clearance phase.

Worth noting is that MUTCD 4D.12 discusses what happens when a signal is to be changed from static service to dynamic service. In that context a light is allowed to change directly from flashing red to steady red.

It is possible to have a three-section signal with right-turn arrows analogous to the left-turn version shown in the table above. The FGA-R can be used to allow a right turn after yielding, and the FRA-R for a right turn after stopping and yielding -- this implements the popular right turn on red. TFGAP when extended to right turns gives a certain flexibility beyond that of conventional traffic signals, because it can prohibit the right turn on red by showing SRA-R during those phases where a hazard exists. Examples:

• When oncoming traffic enjoys a protected left turn; in this case right-turning drivers sometimes neglect to yield to oncoming traffic.
• When traffic from the right enjoys a protected left turn, and is allowed to make u-turns; drivers turning right on red often fail to notice these vehicles.
• When a cluster of utility poles prevents drivers from seeing pedestrians, preparing to cross the street, to whom the drivers would have to yield.

The right-turn arrow will probably never be implemented as widely as the left-turn arrow, because turning right is usually easier and safer than turning left. Instead, vehicles turning right will simply rely upon the conventional dot signal. The standard right turn on SRD is compatible with TFGAP.

Static service is detailed in TFGAP-S, and the MUTCD covers it in its sections 4D.11, 4D.12, and 4K. However, the subject is mentioned briefly on this page in order to discuss compatibility of the new with the old. There are two major varieties of static service:

• Red dots and arrows flash for all drivers in all directions. All drivers must stop for a flashing red signal before proceeding, whether under traditional rules or TFGAP.
• The main road gets a flashing yellow signal and the side road gets a flashing red signal. In this context, an FYA has a much different meaning than in TFGAP. However, drivers should be able to tell the difference, because when the controller is in static service, the FYA continues for minutes or hours. By contrast, under dynamic service with TFGAP the FYA appears for only three to six seconds, after which it turns red.

Some proposals for signaling a permissive turn use a flashing yellow arrow instead of green. But because this light can last much longer than six seconds, a subtle safety problem arises. Illustration:

1. A motorist, who plans to turn right, approaches an intersection where turning left or right is allowed, but going straight is prohibited. All the signals he can see are FYA-Ls and FYA-Rs.
2. He might conclude that the signals are in dynamic service, and that he must yield to other traffic. Alternatively, he might conclude that the signals are in static service, all cross traffic has a flashing red signal, and consequently the cross traffic must yield to him.
3. Not having a clear indication one way or the other, he opts for the second line of thought, making his right turn in the direction of an arrow without looking for cross traffic. Discovering that he was wrong, he crashes into a car that had entered the intersection from the left and was heading straight on an SGA-S.

In TFGAP this uncertainty is avoided, because if an FYA lasts more than six seconds, the traffic lights are surely in static service. In normal TFGAP operation the FYA after just a few seconds changes to an SRA or FRA, either of which requires a stop.

9A. Yellow Trap. TFGAP addresses the extensively-discussed yellow trap problem by giving left-turn traffic and straight traffic completely separate signals. As a result the lagging left-turn protected phase becomes feasible. Here to illustrate is a sequence of signals at an ordinary crossroads. Throughout, eastbound and westbound traffic have red lights.

1. Northbound and southbound thru traffic have SGDs, while northbound and southbound left turn traffic have FGAs. A large number of southbound cars waiting to turn left have accumulated.
2. Northbound thru traffic gets an SYD.
3. Northbound thru traffic gets an SRD.
4. Southbound left-turn traffic gets an SGA. Meanwhile, the southbound thru traffic still has an SGD, and northbound left-turn traffic still has its FGA.

9B. Turns that are Always Prohibited. When certain turns or straight movements are barred due to one-way configuration of streets or other reasons, dot signals are sometimes avoided in favor of arrows pointing in the legal directions of travel, and these arrows under TFGAP would be set to flash whenever drivers must yield to vehicles and pedestrians. When protected turn phases are not implemented, drivers making turns will need FGAs and FYAs because of pedestrians, even if there are no conflicting vehicles. Drivers going straight automatically enjoy protection, so they receive SGAs and SYAs.

A similar principle applies when an intersection is next to a railroad grade crossing, and certain turns or straight movements are prohibited for many minutes while a train is blocking the road. FGAs and SGAs can indicate allowed movements, and SRAs can point in the blocked directions.

An SRA should not be used to inform drivers of a movement that is always illegal, because motorists assume that a steady red signal will eventually change, and they may obstruct traffic for a long time waiting for something to happen. Instead, signs such as ONE WAY or NO LEFT TURN should be posted.

9C. Pre-emption and Priority. In some areas, certain important vehicles can by remote control manipulate traffic lights. Freight trains can clear cars from grade crossings to avoid crashes, and mass-transit vehicles can spend less time waiting for lights to change. Pre-emption and priority control in TFGAP require no change from the principles in MUTCD 4D.13. In particular, when an FGA phase must be terminated the light changes first to an FYA and then to an SRA.

9D. Signals with Fewer Than Three Sections. This topic is now discussed in TFGAP-S.

9E. Combination Lanes. If drivers in the left lane are allowed to either turn left or go straight, some phasing combinations will not be efficient either with or without TFGAP. Examples:

• Straight traffic has an SGD, and left turn traffic has an SRA. If the first driver in the left lane wants to turn left, any drivers behind him who want to go straight are blocked.
• Straight traffic has an SRD, and left turn traffic has an SGA. If the first driver in the left lane wants to go straight, any drivers behind him who want to turn left are blocked.

9F. Drivers with Faulty Color Vision. Motorists who cannot fully distinguish red, yellow and green can rely upon positional cues in the TFGAP left-turn signal, because red is at the top (or left), yellow in the middle, and green at the bottom (or right) -- this is the time-honored positioning. Some other left-turn arrow arrangements use four or five vertically-arranged sections in the cluster, and the colors may not be naturally inferable from the positions.

9G. Uncongested Intersections. There is no need to install a flashing green arrow on every traffic light; the obvious candidates are those intersections where protected left-turn phases have already been implemented but have proven insufficient to prevent backups. In particular, intersections that are now controlled satisfactorily with the ordinary red, yellow and green dots should not be changed.

9H. Other Nations. In Canada there is a tendency to use a flashing green arrow for the protected phase, not the permissive; this is in conflict with TFGAP and United States practice in general. However, the educational sign pictured above should be enough to alert Canadians who are visitng the US to the differing standard; more broadly, drivers ought to realize that traffic laws and driving customs do change when they cross international boundaries. European drivers certainly understand this, with signals varying widely from country to country on that continent.

Signal	MUTCD 4D Reference	Meaning in TFGAP Dynamic Service	Meaning in Static Service
Steady Green Arrow	4D.04.A.2, 4D.05.F, 4D.06, 4D.07, 4D.12	Drivers proceed in the direction of the arrow, and all conflicting traffic is signaled to stop or yield.
Steady Yellow Arrow	4D.04.B.1, 4D.05.E, 4D.06, 4D.07	The SGA phase is ending, and drivers should refrain from entering the intersection if they can stop in time. Drivers who cannot avoid stopping will treat this as an SGA.	None.
Steady Red Arrow	4D.04.C.2, 4D.05.D, 4D.06, 4D.07	Drivers who want to proceed in this direction must stop and wait until the signal changes.	None.
Flashing Green Arrow	None.	Drivers proceed in the direction of the arrow, after yielding to conflicting vehicles and pedestrians.	None.
Flashing Yellow Arrow	Different from 4D.04.D.3	The FGA phase is ending, and drivers should refrain from entering the intersection if they can stop in time. Drivers who cannot avoid stopping will treat this as an FGA.	Drivers proceed with caution in the direction of the arrow. Going straight is protected, but the MUTCD is not clear whether turns require yielding.
Flashing Red Arrow	Similar to 4D.04.D.3	Drivers proceed in the direction of the arrow, after stopping and yielding to conflicting vehicles and pedestrians.
Steady Green Dot	4D.04.A.1, 4D.05.C	Drivers proceed in any direction not otherwise prohibited. Drivers turning left yield to oncoming traffic, and drivers making any turns yield to pedestrians.	None.
Steady Yellow Dot	4D.04.B.1, 4D.05.B	The SGD phase is ending, and drivers should refrain from entering the intersection if they can stop in time. Drivers who cannot avoid stopping will treat this as an SGD.	None.
Steady Red Dot	4D.04.C.1, 4D.05.A	Drivers must stop and wait until the signal changes, except that a right turn is permitted after stop and yield.	None.
Flashing Green Dot	None.	Not used in traditional signaling.
Flashing Yellow Dot	4D.04.D.1, 4D.11, 4D.12	Whether in a warning sign or at an intersection, the FYD tells drivers to use caution. At an intersection, drivers turning must yield to other traffic.	Drivers proceed with caution. Going straight is protected, but turns require yielding unless otherwise indicated.
Flashing Red Dot	4D.04.D.2, 4D.11, 4D.12	Drivers proceed in any direction that not otherwise prohibited, after stopping and yielding to conflicting vehicles and pedestrians.

Notes:

• Static service is detailed in this table to demonstrate that TFGAP dynamic service is compatible with it.
• A driver need not know in advance whether a signal offers TFGAP service. If he does get the FGA he may attempt a permissive turn, but if the FGA never appears he simply follows the same traffic rules that he always has.
• The use of an FGA is not mentioned in MUTCD, nor is the use of an FYA to indicate the end of permissive turn phase. The rule pertaining to FRAs might need subtle change to reflect who yields to whom after a driver stops: what happens in static service may differ from what happens when some drivers get the FRA and others have steady green signals.
• The straight red or yellow arrow is currently prohibited by MUTCD 4D.8.

11A. Tee Intersections. Consider an intersection with an east-west main road and a side road to the south, all two-way streets. Ordinarily, when traffic from the south has a green signal that allows a left turn, traffic from the east has a steady red signal. An alternative is to give the traffic from the east an FGA-S or FRA-S so drivers could proceed with caution when no vehicles were coming from the left and no pedestrians were in the way.

11B. Pedestrian Signals have been moved t0 TFGAP-P.

11C. Static Service has been moved to TFGAP-S.

11D. Beacons have been moved to TFGAP-B.

11E. U-turns. Some divided highways in commercial zones are burdened with a large number of driveways into the parking lots of retail businesses. A motorist heading to or from one of these establishments often finds himself on the wrong side of the road and must make a u-turn to complete his trip. In areas where the number of u-turns is overwhelming it may be appropriate at an intersection to construct, to the left of the left-turn storage lane, a u-turn storage lane with its own signal. Below is a version of the light similar to what has been tested in at least one locality.

Depending on the phase, the left-turn and u-turn signals might sometimes be of different colors. For example, when vehicles coming from the south have a protected left turn and vehicles approaching from the west have a protected right turn, motorists approaching from the south and planning to make a u-turn must not have protection because of a conflict with the right-turners from the west. These u-turners could get a permissive signal if the planners judge that such turn can be safe; otherwise an SRA-U is in order.

The u-turn arrow is also appropriate if the cross street is one way from left to right, and the left turn must be prohibited.

Intersection geometry often dictates a tight radius for u-turns, because they are customarily made from the leftmost lane -- trucks can have great difficulty with this. This u-turn arrow permits special signal phases that allow intersection designers to place the u-turn lane in an extraordinary location, such as to the right-hand side of the left-turn lanes.

11F. The Flashing Pattern. Most traffic signals have one light bulb per section, so that section when flashing must be entirely on or entirely off at any instant. For instance, when this image below left was shown in section 4 it was merely a shorthand for the pair of alternating dark and light aspects below right. This can readily be implemented in mature technology.

The increasing application in recent years of light-emitting diodes to traffic signals gives designers more options. Following are several other ways that flashing might be effected by alternation between a pair of aspects. The operative section is always lit at least in part, assuring that a driver will never see an ambiguous dark signal when taking quick glances around an intersection to evaluate the traffic and lights.

As electronic research continues, all sorts of animation will become feasible.