LEFT TURN SIGNALS

OLD AND NEW

VARIOUS LEFT TURN SIGNALS AND IMPLEMENTATIONS

Left turn signals have been in use since the early 1920s. Some of them have been rather unusual in appearance and operation (one version let turns go in all four directions simultaneously). But by 1950, standard methods emerged, and the methods have been refined over the years since then. Here is a history of the various methods, along with the details of how they worked:

TABLE OF LEFT TURN SIGNALS

KEY:

• Red text: Hazard or illegal implementation
• Red background: Dangerous portion of sequence
• Opposing Signal: The opposing signals are signals facing oncoming traffic in front of the motorist, presented as if seen in a mirror
• With two exceptions, each signal implementation is shown in a lead-lag phasing (the most difficult to implement safely). Other phasings are possible.
• MUTCD = Manual Of Uniform Traffic Control Devices for Streets and Roads, US Department of Transportation
• Yellow Trap: Traffic receiving a circular yellow while the opposing light still has a circular green. (more)
• Except for the flashing speed of the arrows, the animation is running about double the normal speed.

Opposing Signal (mirror image)	Signal Display as seen by motorist	Name	Uses	Conditions
		1. NO CLEARANCE No clearance periods between turn arrow and opposing thru.	Before 1961	DANGER: No clearance interval DANGER: Yellow trap Prohibited by MUTCD (1, 2, 3).
		2. BLIND CLEARANCE Clearance intervals, but no clearance indications.	Typical before 1971	CAUTION: No clearance indication DANGER: Yellow trap Prohibited by MUTCD (1, 3).
		3. CIRCULAR CLEARANCE Clearance intervals use same circular yellow.	Rare, all before 1961	CAUTION: Confusing clearance indication DANGER: Yellow trap Prohibited by MUTCD (1, 4).
		4. DOGHOUSE PERMISSIVE Separate clearance indications	Ubiquitous after 1971 Inline 5-light also used	DANGER: Yellow trap Trap prohibited by MUTCD (1). New standard for shared lanes
		5. EXCLUSIVELY PROTECTED* TURNS Separate signal faces	Ubiquitous after 1961 (Yellow & red arrows after 1971)	Inefficient No yellow trap
		6. FREE-FLOW T-INTERSECTION Free-flow movements	Billerica Massachusetts (discontinued)	T with merge lanes Doghouse not using sections is clearer. Prohibited by 2009 MUTCD (8, 9).
		7. USES 5-LIGHT YELLOW ARROW Permissive and clearance use same lens.	Sparks Nevada had similar use Inline 5-light also used	Clearance & permissive confused. Prohibited by 2009 MUTCD (5).
		8. USES 3-LIGHT CIRCULAR RED Stop and permissive use same lens.	Michigan Maryland	Requires stop before turn. Prohibited by 2009 MUTCD (10, 11).
		9. USES 3-LIGHT CIRCULAR YELLOW Permissive and clearance use same lens.	Washington State (discontinued)	Clearance, permissive, & thru confused. Prohibited by 2009 MUTCD (6, 7, 11).
		10. DOUBLE CIRCULAR YELLOW Special flashing circular yellow for permissive.	Seattle Washington	Permissive turn and thru confused. Prohibited by 2009 MUTCD (7).
		11. 4-LIGHT - ADDS RED ARROW Flashes extra red arrow.	Delaware, Maryland, Cupertino California	Requires stop before turn. New 2009 MUTCD flashing red arrow is similar
		12. USES 3-LIGHT YELLOW ARROW Stop and permissive use same lens.	Jackson County Oregon Scottsdale Arizona	Clearance & permissive confused. Prohibited by 2009 MUTCD (6).
		13. "FLASHING YELLOW ARROWS" (FYA) Special yellow arrow for permissive	Reno Nevada New 2009 MUTCD	New MUTCD P/P standard. Permissive & clearance confused at night.
		14. 5-LIGHT FYA Separate lenses for each function	Broward County Florida Idaho (plus all-arrow variant)	Optimum case Prohibited by 2009 MUTCD (8, 11).
		15. FYA CIRCULAR CLEARANCE Clearance intervals use same circular yellow.	Not yet used	Optimum case Prohibited by 2009 MUTCD (8, 11).
		16. 5-LIGHT DUAL-COLOR ARROW Shared doghouse with FYA	Seattle Washington Tucson Arizona (variant)	Shared use confusing. Prohibited by 2009 MUTCD (5).
		17. DALLAS PHASING As seen from left turn lanes. Uses signal louvers.	Dallas Texas Other sites In 2003 MUTCD	Circular indications louvered. No span wires. Prohibited by 2009 MUTCD (8, 11).
		18. DALLAS PHASING As seen from straight ahead lanes. Uses signal louvers.	Dallas Texas Other sites In 2003 MUTCD	(This is a view of #17 from a thru lane.) Prohibited by 2009 MUTCD (8, 11).
		19. 4-LIGHT CLUSTER FYA (Opposing) ----- 3-LIGHT PERMISSIVE FYA (Motorist's display)	Not yet used ----- 2009 MUTCD permissive face	For height limit. Prohibited by 2009 MUTCD (12). ----- Opposing left pro­tected, not motor­ist's.
		20. 3-LIGHT DUAL ARROW FYA Uses dual-color arrow.	Kennewick Washington Bend Oregon	Intended for height limit
		21. SWISS P/P Uses flashing yellow triangle.	Switzerland	Flashing triangle modifies green arrow to make it permissive. Not in MUTCD.
		22. FRENCH P/P Uses + display to show oncoming yellow and red indications.	France	Drivers in intersections can't see regular signals. The + signals tell them when oncoming traffic is stopped. Not in MUTCD.

MUTCD NOTES

• Note (1): The 2003 (and later) MUTCD requires yellow trap to be eliminated or warned with a sign. The grace period expired in 2008.
• Note (2): The 1961 (and later) MUTCD requires a clearance period after each left turn movement.
• Note (3): The 1978 (and later) MUTCD requires a VISIBLE clearance indication after each left turn movement.
• Note (4): The 1935 (and later) MUTCD prohibits simultaneously illuminating two circular lenses of different color in the same face.
• Note (5): The 2009 MUTCD prohibits Flashing Yellow Arrows in shared-use signal faces, or where lanes share left and thru movements.
• Note (6): The 2009 MUTCD prohibits using the same yellow lens for both permissive turns and clearance.
• Note (7): The 2009 MUTCD prohibits using a flashing circular yellow for permissive left turn. This usage was experimental.
• Note (8): The 2009 MUTCD prohibits a circular green indication over or in front of the left turn lane.
• Note (9): The 1961 MUTCD (and later) requires a green arrow in a single section face for continuous movement.
• Note (10): The 2009 MUTCD prohibits a flashing circular red indication for a permissive turn.
• Note (11): The 2009 MUTCD prohibits a circular red indication in an exclusive left turn signal face.
• Note (12): The 2009 MUTCD prohibits a horizontal cluster of green arrow and flashing yellow arrow pointing the same direction in a vertical face.

HISTORY OF LEFT TURN SIGNAL IMPLEMENTATION

Early Left Turn Signals:

In most of the 1950s and 1960s, left turn signals consisted of a single green arrow attached to a standard 3-section face as a 4th section. They were operated in various ways, because there were no uniform standards of operation. Examples 1, 2, and 3 are signals of this type:

Version 1 was still used until about 1969, even though the standards prohibited it in 1961. It had the following characteristics:

• There was no clearance interval between the leading left turn phase and the oncoming thru phase.
• Clearance was accomplished by the law requiring drivers to yield to vehicles already in the intersection.
• A few installations put the green arrow at the top of the signal face.

Version 2 was used into the mid 1970s. The 1971 MUTCD provided a better alternative, and the 1978 MUTCD prohibited this display. Its properties were:

• There was a clearance interval between the leading left turn phase and the oncoming thru phase.
• The clearance was obtained by turning off the green arrow. The yellow arrow was not in general use until after 1971, but several nonstandard experimental uses were implemented (see version 4).
• Indianapolis Indiana used a nonstandard version of this with a yellow arrow instead of a green arrow, because the intersections had no pedestrian signals. The leading left turn had a pedestrian conflict.
• A few installations put the green arrow at the top of the signal face.

Version 3 was rare, and all uses were gone by 1964 (The 1971 MUTCD provided a replacement). It had the following characteristics:

• There was a clearance interval between the leading left turn phase and the oncoming thru phase.
• The clearance was obtained by turning off the green arrow and lighting the circular yellow. This caused confusion, because Pennsylvania uses a display of circular green with circular yellow as a pre-clearance warning.

All of the preceding signal types used signs to differentiate the intended type of phasing: The signs are:

• "LEFT TURN ON ARROW ONLY" * - exclusively protected* left turns
• "LEFT TURN GREEN OR ARROW" - protected-permissive left turns

No attempts to prevent Yellow Trap: were made at this time, other than prohibiting opposing left turns or avoiding lagging green arrows.

Mature Left Turn Signal Designs:

Version 5 was actually next, introduced as a standard in 1961. It quickly became the standard for exclusively protected* phasing, and still is. But the earliest version was different:

• The original signal had a green arrow, circular yellow, and circular red.
• 1971 standards introduced the yellow arrow for clearance, and the red arrow for stop. Circular red could be substituted for red arrow.
• This was the first display able to prevent yellow trap. But it does not prevent yellow trap for single-direction lag left turns.
• The 2009 MUTCD now requires the use of the red arrow for all exclusively protected* displays.

Version 4 appeared in the mid 1960s, though it was nonstandard until 1971. Clustering was also first allowed in 1971. By the late 1970s, it became the standard protected-permissive display:

• The 1978 MUTCD prescribes a yellow arrow clearance interval following each green arrow phase.
• This display allows yellow trap to occur.
• 1971 standards introduced the yellow arrow for clearance.
• This face may also be made as a vertical column or a horizontal row.
• The 2003 MUTCD requires that yellow trap must be either prevented or warned, with a 5-year compliance period.
• The 2009 MUTCD prohibits a circular green in an exclusive left turn display.
• The 2009 MUTCD prohibits placing Version 4 faces left of the lane line between the left turn lane and the straight ahead lanes.
• Version 4 may soon be removed as the standard display for protected-permissive phasing, because it can not prevent yellow trap.
• Version 4 may become the standard display for shared use lanes (left and thru), but with a different sequence.

Version 6 is a special signal for use at a T intersection where the thru movement is not required to stop. Entering traffic merges with the thru movement after the signal. A better implementation could use the displays from version 4, without lighting the circular yellow and circular red lenses. The 2009 MUTCD prohibits a circular green in an exclusive left turn signal. A single section circular green has never been allowed. The MUTCD requires a green arrow.

Early Efforts to Prevent Yellow Trap:

Version 4 is now used with special phase inhibiting logic to prevent yellow trap. But this requires that both left turns be either leading or split phase (unsplit lead-lag).

The following displays were designed to make it possible to continue the permissive turn phase after the adjacent thru phase turns red. This is necessary to prevent yellow trap. This cannot be done with circular green, without special visibility limiting devices.

Version 7 has been used in a few places. An experimental implementation was made in the early 1960s. But it can cause confusion:

• Since the same yellow arrow is used for both permissive turns (flashing) and clearance (steady), it is hard to tell when the clearance period starts.
• Early implementations were intended to show the need to yield, not to prevent yellow trap. They were phased wrongly, so they could not prevent yellow trap.
• Sparks Nevada used a version where the circular green was always lit with the flashing yellow arrow. But this could not prevent yellow trap.
• The 2009 MUTCD prohibits the flashing yellow arrow in a shared use display, and requires that the clearance yellow arrow must be in a different section.

Version 8 was first used in 1975 in Michigan. Maryland later started using it:

• The circular red lens of a signal face otherwise used for exclusively protected* phasing is flashed for permissive turns.
• Each driver turning on a flashing red must stop before turning.
• This display was the first to automatically prevent yellow trap with protected-permissive phasing.
• The clearance interval is blind - the red stops flashing at the beginning of the clearance.
• The 2009 MUTCD discourages the use of a flashing red for permissive left turn, but does not prohibit it. But it must be a red arrow.
• The 2009 MUTCD requires a yellow arrow clearance when changing from flashing red arrow to steady red arrow.
• Michigan is now replacing these with version 13.

Version 9 has been in extensive use in Washington State:

• Since the same yellow lens is used for both permissive turns (flashing) and clearance (steady), it is hard to tell when the clearance period starts.
• The 2009 MUTCD requires a second lens for the clearance indication.
• Washington State is in the process of replacing these with version 13.
• The 2009 MUTCD prohibits this display.

Version 10 was used in Seattle Washington since 1966:

• The original signal had a green arrow in a vertical 4-section face. The same circular yellow cleared the flashing circular yellow and the green arrow. The only change from this face to Version 13 is replacing the signal lenses with arrows.
• The second version had a 5-section face, often vertical.
• The latest design has a dual-color arrow in a vertical 4-section face.
• Washington State is changing these to version 13.
• The 2009 MUTCD prohibits this display. It was an experimental usage.

Version 11 was used in Delaware, Maryland, and in Cupertino California:

• The Delaware signal has a cluster, with the red arrow to the left of the circular red. The other indications are arrows below the cluster.
• The Maryland and California signals use the lenses in the vertical inline arrangement shown.
• Each driver turning on a flashing red must stop before turning.
• The 2009 MUTCD discourages the use of a flashing red arrow for permissive left turn, but does not prohibit it.
• The 2009 MUTCD specifies a flashing red arrow face like this one, but without the circular red lens. The face looks like the face in version 5, but works like this version. The same red arrow lens is used for both red indications. Or two red arrows may be used, one for each indication
• The 2009 MUTCD requires a yellow arrow for clearance when the flashing red arrow changes to steady red arrow.
• The 2009 MUTCD allows the circular red lens to be replaced with a second red arrow in either arrangement.

Recent Efforts to Prevent Yellow Trap:

Version 12 was used in Jackson County Oregon:

• The yellow arrow lens of a signal face otherwise used for exclusively protected* phasing is flashed for permissive turns.
• The same yellow arrow flashes for permissive turn, and is steady for clearance.
• Since the same yellow arrow is used for both permissive turns (flashing) and clearance (steady), it is hard to tell when the clearance period starts.
• The 2009 MUTCD prohibits this display, requiring a different yellow arrow lens for clearance.

Flashing Yellow Arrows:

Version 13 was first used in Reno Nevada. This Flashing Yellow Arrows face may soon become the standard left turn signal:

• The 2009 MUTCD introduces this as a face for use with any kind of left turn phasing. But there must be a separate left turn lane.
• General-use interim approval for Flashing Yellow Arrows was available before the 2009 MUTCD was released.
• Since yellow arrows are used for both permissive turns (flashing) and clearance (steady), it is hard to tell when the clearance period starts at night (when the dark parts of the signal face can't be seen). A yellow border on the signal backplate alleviates this.
• When properly implemented, the Flashing Yellow Arrow face can always prevent yellow trap.
• When improperly implemented, this becomes dangerous. Implementation is not a job for layman politicians.
• Care must be used when pre-empting signals, or yellow trap will occur. The opposing yellow arrow must flash during fire or rail preemption.
• This probably will soon become the universal left turn signal for all left turns from exclusive lanes.
• Several states are converting all protected/permissive signals with left turn lanes to Flashing Yellow Arrows.
• Some Texas locations converted 5-section Dallas Phasing to Flashing Yellow Arrows by using two red arrows lighted simultaneously.
• This signal face can also display a flashing red arrow if needed.
• Care must be taken to prevent Second Yellow Trap.
• This signal face can be changed by a timer, computer, or conditions present among six modes:
  1. Exclusively protected* mode
  2. Protected-permitted mode using a flashing yellow arrow
  3. Protected-permitted mode using a flashing red arrow
  4. Exclusively permitted* mode using a flashing yellow arrow
  5. Exclusively permitted* mode using a flashing red arrow
  6. prohibited* mode (e.g. for railroad pre-emption, to keep traffic off the tracks)

Version 14 is a 5-light version of version 13:

• This was first used in Broward County Florida, but in a 5-lens vertical arrangement.
• Idaho also used it, in the doghouse configuration as shown.
• Pocatello Idaho had a variant with all arrows, but the same lens layout.
• The 2009 MUTCD allows the Pocatello all arrows version, but prohibits circular red in exclusive left turn faces.
• In each case, the flashing yellow arrow was placed where the circular green lens is normally found in a doghouse.
• It is easier to implement this than the 4-light version 13. Less signal logic is needed.
• It removes the possibility of confusing the flashing permissive indication with the clearance indication, by using a different lens shape.
• When properly implemented, the Flashing Yellow Arrow face can always prevent yellow trap.
• When improperly implemented, this becomes dangerous. Implementation is not a job for layman politicians.
• Care must be used when pre-empting signals, or yellow trap will occur. The opposing yellow arrow must flash during fire or rail preemption.
• This could have been the universal left turn signal for all left turns from exclusive lanes if allowed.
• Care must be taken to prevent Second Yellow Trap.
• This signal face can be changed by a timer or computer among three modes:
  1. Exclusively permitted* mode
  2. Exclusively protected* mode
  3. Protected-permitted mode

Version 15 is a version of version 13 with two arrows replaced by circular indications:

• It is implemented with the same logic required for version 13. Only the lenses are different.
• It removes the possibility of confusing the flashing permissive indication with the clearance indication by using a different lens shape.
• The 2009 MUTCD prohibits circular indications in exclusive left turn faces.
• When properly implemented, the Flashing Yellow Arrow face can always prevent yellow trap.
• When improperly implemented, this becomes dangerous. Implementation is not a job for layman politicians.
• Care must be used when pre-empting signals, or yellow trap will occur. The opposing yellow arrow must flash during fire preemption.
• This could have been the universal left turn signal for all left turns from exclusive lanes.
• Care must be taken to prevent Second Yellow Trap.
• This signal face can be changed by a timer or computer among three modes:
  1. Exclusively permitted* mode
  2. Exclusively protected* mode
  3. Protected-permitted mode

Version 16 was proposed as an alternate way to use Flashing Yellow Arrows with a shared signal face:

• The original NCHRP (National Cooperative Highway Research Program) report suggested this as a shared signal face.
• Tucson Arizona had a variant that exchanged the use of the two yellow arrows. Police demanded its removal after an accident.
• The 2009 MUTCD allows a dual-color arrow for use with Flashing Yellow Arrows.
• The 2009 MUTCD prohibits Flashing Yellow Arrows in a shared signal face.

Dallas Phasing:

Version 17 is the Dallas Phasing system. It was first used in Dallas Texas. There are problems with implementation:

• The circular green and yellow indications must be louvered or optically programmed, so thru movement drivers can't easily see them.
• The necessity for accurate aiming causes problems with span-wire installations and swivel mounts.
• This display was the first specifically designed to prevent yellow trap in lead-lag phasing.
• The 2003 MUTCD allows the use of these displays.
• The 2009 MUTCD prohibits this display. It says circular green should not be used in an exclusive left turn signal.
• Existing installations are allowed to remain in operation until replaced.
• Care must be taken to prevent Second Yellow Trap.
• The version 18 picture is the same signal, but shows the display as seen from the thru lanes (where louvers hide some of the indications).
• Arlington phasing is like Dallas phasing, except that the left turn on the lagging side is not given a permissive green during the leading left turn.

Alternate and Permissive Signal Faces:

Version 19 shows two different displays. This is necessary to show the use of the permissive display:

1. 19-A The display shown to opposing traffic is a cluster version of version 13:
   • This is functionally identical to version 13. The only difference is clustering the green arrow and flashing yellow arrow.
   • The 2009 MUTCD specifically prohibits the use of this display.
   • This version reduces the possibility of confusing the flashing permissive indication with the clearance indication, because the indication changes diagonally.
   • This display could be used for reduced clearance locations, rather than version 20. But the 2009 MUTCD does not allow it.
   • Care must be taken to prevent Second Yellow Trap.
   • See version 13 for the other properties.
2. 19-B Exclusively Permissive*: The display shown to the motorist is the new display for preventing yellow trap where no left turn phase exists for the approach:
   • This is the standard 2009 MUTCD display for approaches without left turn phases that can otherwise produce a yellow trap.
   • The flashing yellow arrow flashes whenever the opposing thru signal is green.
   • The signal face can usually be wired to the opposing signal face without any extra logic except flashing.
   • This display was the first to prevent yellow trap for exclusively permissive* displays.
   • Care must be taken to prevent Second Yellow Trap.
   • This display must be used where any exclusively permissive* turn is exposed to yellow trap.

Version 20 is an alternate way to use Flashing Yellow Arrows with a 3-section signal face:

• The original NCHRP report suggested this as a 3-section signal face for reduced clearance locations.
• It has been used in Kennewick Washington and Bend Oregon.
• The 2009 MUTCD allows a dual-color arrow for use with Flashing Yellow Arrows.
• Care must be taken to prevent Second Yellow Trap.
• The display shown in Version 19-A performs the same function.

Version 21 is a European version of a protected/permissive signal face that prevents yellow trap:

• The original NCHRP report mentions it.
• It has been used in Switzerland for years.
• The MUTCD has no such definition.
• Care must be taken to prevent Second Yellow Trap.
• This performs the same function as the display shown in Version 13.

Version 22 is a European version of protected/permissive signal faces that together prevent yellow trap:

• It is not mentioned in any US report.
• It has been used in France for years.
• The MUTCD has no such definition.
• Care must be taken to prevent Second Yellow Trap.
• This is closest to Arlington Phasing.
• The main signals are not visible to vehicles waiting in the intersection to turn left.
• Drivers in the intersection are told by the + signal when oncoming traffic receives a yellow, and then a red.
• The + display could be used without left turn phases on the approach to prevent yellow trap from a left turn phase on the oncoming approach.

Prediction:

I predict that eventually we will eventually have only 4 left turn signal faces, plus a face for a signal with no special sequences:

1. Version 13 or 20 will be used for most left turn faces where left turn lanes and left turn phases exist.
2. Version 4 (or 2) will be used for lanes sharing left and thru movements. No phasing except split phases (unsplit lead-lag) can be used.
3. Version 5 will be used for multiple left turn lanes and some intersections with unusual geometries.
4. Version 19-B will be used for approaches with left turn lanes, but no left turn phases. It might also be used at 2-phase signals.
5. The face with all circular indications will still allow left turns where no left turn phases or pre-emption sequences exist on a street.
6. Other methods for preventing yellow trap must be used where exclusive left turn lanes do not exist.

Maybe someday, we will have this true uniformity. Then drivers will know what to expect.

Links:

• All Yellow Trap Links are here:
  YELLOW TRAP INDEX PAGE
• How Left Turn Signals Work
• A visual example of Flashing Yellow Arrows
• DO YOU KNOW WHAT YELLOW-TRAP IS?
• Unusual Traffic Problems and Solutions
• Home