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:

Oncoming

Signal

(mirror

image)

 
 
Signal Display
as seen by motorist

 
 
 
Name

 
 
 
Uses

 
 
 
Conditions

signal diagram

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.

Pennsylvania & Japan have.
Some other states before 1961 (rare)

CAUTION: Confusing clearance indication
DANGER: Yellow trap
Prohibited by 1961 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
Safest sequence.

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
Virginia
Florida

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).
All-arrow variant allowed.

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 is confusing.
Prohibited by 2009 MUTCD (5),
but allowed by FHWA interpretation
with lane use control, or for a shared
lane with variable left turn mode.

17. DALLAS PHASING
As seen from left turn lanes.
Uses signal louvers.

Dallas Texas
13 other states used

Circular indications louvered.
No span-wire mounts.
Allowed by 2003 MUTCD.
Prohibited by 2009 MUTCD (8, 11).
Existing signals grandfathered.

18. DALLAS PHASING
As seen from straight ahead lanes.
Uses signal louvers.

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
In 2009 MUTCD

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

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:

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:

Version 3 was rare, and all uses were gone by 1964 except in Pennsylvania, where it is still in use. The 1971 MUTCD finally provided a useful replacement for it. It had the following characteristics:

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

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:

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:

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:

Version 8 was first used in 1975 in Michigan. Maryland later started using it. It was also used in a few places in Virginia and Florida.:

Version 9 has been in extensive use in Washington State:

Version 10 was used in Seattle Washington since 1966:

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

Recent Efforts to Prevent Yellow Trap:

Version 12 was used in Jackson County Oregon:

Flashing Yellow Arrows:

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

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

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

Version 16 was proposed as an alternate way to use Flashing Yellow Arrows with 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:

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:
  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:

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

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

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

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.

* The standard phrases in use violate the rules in English for placing the word "only," or lack clarity.

The word "only" always modifies the word AFTER it (never the word before it). "Only" is NOT an adverb, so it does not follow the verb. The following table shows the wrong phrases, and their proper replacements:

MUTCD phraseReplacement

protected-only

exclusively protected

permitted-only

exclusively permitted

dual arrow

dual color arrow

LEFT TURN ON ARROW ONLY

LEFT TURN WAIT FOR ARROW

LEFT TURN ONLY

MUST TURN LEFT

LEFT TURN ONLY THIS LANE

LEFT LANE MUST TURN LEFT

Links: