Leading and Lagging
Left Turn Signals Compared

Now that the yellow trap problem can be removed, which is better?

By Larry Robinson

There is a lot of confusion and controversy about left turn phasing. This page is intended to remove some of the confusion.

Terminology

Even the terminology about left turn phasing is confusing, because so many different terms have been used for the same things, and because some of the terms and abbreviations have been used for different things. A complete glossary is provided at the end of the article in Appendix A. For immediate use, here is a list of the unambiguous terms used in the rest of this article:

Circular green permissive - Permissive left turns made during a circular green indication.
Double-Split Lead-Lag - One leg on the street has leading turn, opposite leg has lagging left turn. Either phase can change first.
Dual Lag - Both left turns on the street have simultaneous lagging turn phases.
Dual Lead - Both left turns on the street have simultaneous leading turn phases.
Dual Split Lag - Both left turns on the street have lagging phases. They can begin at different times.
Dual Split Lead - Both left turns on the street have leading phases. They can end at different times.
Exclusively/Permissive - No left turn phase. Left turns filter through gaps in oncoming traffic.
Exclusively/Protected (E/P) - The left turn is protected during one phase, and is not allowed to proceed otherwise.
Flashing Yellow Arrow - An indication where left turns are made through gaps in oncoming traffic.
Flashing Yellow Arrow permissive - Left turns made through gaps in oncoming traffic during a flashing yellow arrow indication.
Lagging left turn - A sequence where the green left turn arrow is next after the oncoming thru green.
Leading left turn - A sequence where the oncoming thru green is next after the green left turn arrow.
No-Split Lead-Lag - Left turn phasing where each leg on the street has an exclusive phase.
Permissive left turn - An indication where left turns are allowed to filter through gaps in oncoming traffic.
Prohibited left turn - An indication where left turns must wait until the indication is no longer displayed.
Protected left turn - An indication with a green left turn arrow, where no other traffic can move in conflict with the turn.
Protected/Permissive (P/P) - The left turn is protected during part of the cycle and permissive during another part.
Single lag - A green left turn arrow is displayed in one direction next after the oncoming thru green.
Single lead - The oncoming thru green is displayed next after a green left turn arrow for one leg.
Single-Split Lead-Lag - One leg on the street has leading turn, opposite leg has lagging left turn. The leading turn must end first.
Yellow trap - Left turns trapped in the intersection with a yellow or red light while oncoming traffic is still moving with a green.

Background

An early study made by Harvey Hawkins (A Comparison of Leading and Lagging Greens in Traffic Signal Sequences) in 1963 was one of the first real attempts to compare leading and lagging left turn phasing. But the study was hampered by the following limitations:

The yellow arrow had not yet been added to the MUTCD.
The only way to end a leading P/P left turn at the time was a blind clearance (the green arrow goes out to begin the change interval).
Pedestrian signals were new, and were not often used.
The only ways to deal with yellow trap in 1963 were:
- Prohibiting the opposing left turn
- Simultaneous dual lag phases
- Giving the opposing turn a protected leading phase
- Not using lag left turns.

The only left turn phasings actually studied were leading P/P and lagging P/P. This affected the advantages and disadvantages listed in the study. The advantages and disadvantages from the Hawkins study are paraphrased in the table below with the identifier (H).

The study also examined signs used with leading and lagging left turn phasing:

DELAYED GREEN WAIT (Lead, opposing direction)
WAIT FOR YOUR GREEN LIGHT (or WAIT FOR GREEN LIGHT)
MOVE ON GREEN ONLY
LEFT TURN SIGNAL *
NO LEFT TURN (lag, opposing direction)
WAIT DELAYED SIGNAL (Lead, opposing direction)
3-WAY SIGNAL
LEFT TURN ON ARROW ONLY * (E/P)
LEFT TURN ON YELLOW WITH CAUTION (Lag, opposing direction)
WAIT FOR YOUR GREEN
MOVE ON YOUR GREEN ONLY
DELAYED GREEN
MOVE ON ARROW ONLY (E/P)
DIRECTION CONTROL - OBEY YOUR SIGNAL
WAIT FOR GREEN
LEFT ON ARROW ONLY * (E/P)
OBEY YOUR SIGNAL ONLY *
ADVANCE GREEN
WAIT HERE FOR YOUR GREEN LIGHT
LEFT TURN ON ARROW

Several more signs have been added in the intervening years:

LEFT TURN GREEN OR ARROW
LEFT TURN YIELD ON GREEN _• *
LEFT TURN ON GREEN ARROW ONLY *
LEFT TURN YIELD ON FLASHING YELLOW ARROW *
LEFT TURN ON FLASHING RED AFTER STOP *
LEFT TURN ON FLASHING RED ARROW AFTER STOP *
ONCOMING TRAFFIC HAS EXTENDED GREEN * (Lag, opposing direction)
ONCOMING TRAFFIC MAY HAVE EXTENDED GREEN * (Lag, opposing direction)

* Sign still in use
_• A circle of the correct color on the sign

An article by Benjamin McKay in 1966 (published Lead and Lag Left Turn Signals Traffic Engineering April 1968 pp50-57) was also limited by the following:

The yellow arrow had not yet been added to the MUTCD.
The only way to terminate the leading P/P left turn in 1966 was a blind clearance.
The author thought blind clearance was dangerous, and precluded using it in the article.
The only way to have a leading left turn with yellow clearance was a separate signal face with green arrow, circular yellow, and circular red.
The only ways to deal with yellow trap in 1968 were:
- Prohibiting the opposing left turn
- Simultaneous dual lag phases
- Giving the opposing turn a protected leading phase
- Not using lag left turns.

Thus, the only left turn phasings actually examined were leading E/P and lagging P/P. This affected the advantages and disadvantages listed in the study. The advantages and disadvantages from the McKay article are paraphrased in the table below with the identifier (M). The article concluded that the main requirements for safety were making lead phasings E/P, and avoiding yellow trap with lag phasings. It also suggested choosing lead or lag based on progression advantages.

Another study completed in 1989 was An Evaluation of Leading Versus Lagging Left Turn Signal Phasing, by Hummer, Montgomery, and Sinha. Items from this study are paraphrased in the table below with the identifier (S).

In several places, they do not seem to have realized that the Hawkins study covered only the P/P leading case and the P/P lagging case, and was mainly a collection of opinions from engineers. The following statements made in their study are not true:

"Hawkins [1963] ... stated that, 'Less time (is) needed for the lag since left turns can filter through (on) the straight through indications." However, Hawkins did not provide any supporting data and did not elaborate on his claim.
"Hawkins also pointed out that the protected-permissive signal has a relative safety advantage in reducing the number of potential left and opposing traffic conflicts, since more vehicles presumably turn on the green ball with permissive-protected signals. The conclusions drawn by Hawkins were not supported by factual data."

The advantages and disadvantages listed were provided by the survey respondents. They were not intended to be conclusions. From the Hawkins study:

"The listed advantages and disadvantages for both leading and lagging green intervals were taken from the questionnaires submitted by traffic engineers. They are not verified as valid statements by the committee since they were not reported as based on field research, but are listed as items which may be explored by a subsequent committee on leading and lagging green intervals.

The results of the Hummer study:

Policy should allow more than one sequence for progression purposes.
Yellow trap must be eliminated.
Motorists prefer leading left turns.
Lagging lefts have fewer pedestrian conflicts.
Lagging lefts have fewer left vs. opposing thru vehicle conflicts.
Fewer vehicles run red lights with lagging lefts.
Leading lefts have fewer cases of indecision.
Provided that yellow trap is eliminated, lagging lefts have fewer accidents and fewer injuries.
Lagging P/P lefts have fewer stops per vehicle.
More vehicles turn during the permissive portion of a lagging left than for a leading left.
More vehicles turn during change intervals for a leading left than for a lagging left.
There is no difference in total delay between leading lefts and lagging lefts.
The biggest reduction in delay is caused by the signal sequence with the best fit to the progression plan.
The E/P phasings have more overall delay than the P/P phasings, which have more overall delay than permissive turns with no left turn phases.
For E/P phasing, there is no difference in delay or number of stops between lead and lag.
For simultaneous phasing, lag phasing has less total delay than lead phasing.
The ability to split the lead phasing (overlapping phases) removes the delay disadvantage listed above.
Lagging left turns work better at isolated diamond interchanges.

Comparisons of Sequences

Items added by this page author have the identifier (R).

CHARACTERISTIC ADVANTAGES	NO LEFT PHASE	LEAD P/P	LAG P/P	LEAD E/P	LAG E/P
Permits higher intersection capacity on 2-lane roads (H)		√
Reduces congestion by clearing left turns first (H)		√		√
Uses fewer intervals in the signal cycle (H)		Note X
Easier to adjust to minimum time in off peak periods (H)		Note X
Eliminates left turn conflicts by clearing left turns first (H)		√		√
Driver reaction time is quicker (H)		√		√
Requires only one amber clearance interval (H)		Note X
Desirable where left turn lanes do not exist (H)		√		√
Can be used for progression where unequal spacings exist (H M)		√	√	√	√
Conforms to normal right of way law (H M)			√		√
Close to normal driving (H)			√		√
Does not trap left turn driver when arrow ends (H)			√	√	√
Separates pedestrians and left turns (H)			√		√
Less time for left turns, since they can turn on green before (H M)			√
Left turns don't steal time from thru vehicles when arrow ends (H M)			√	√	√
Cuts off only platoon stragglers (H)			√		√
Desirable where left turn lane exists (H)	√	√	√	√	√
CHARACTERISTIC DISADVANTAGES	NO LEFT PHASE	LEAD P/P	LAG P/P	LEAD E/P	LAG E/P
Pedestrian conflicts with left turns (H, Note D)		√		√
Left turns pre-empt right of way when arrow ends (H M, Note X)		√		√
Opposing movement may make a false start (H)		√		√
Turn ending early lets platoon reach next light early (H)		√		√
Does not conform to normal right of way rule (H M)		√		√
Can be used where no left turn lane exists (R)		√	√
Obstructs thru movement where no left turn lane exists (H)			√		√
Single left turn phase causes yellow trap in normal cycle (H M R, Note A)			√		√
Double left turn phases cause yellow trap in normal cycle (H M R, Note A)			√
Phase skip causes Yellow Trap (H M R, Note A)		√	√
Preemption causes Yellow Trap (H M R, Note A)	√	√	√	√	√
Difficulty of terminating on high speed approach (R)			√		√
CHARACTERISTIC COMPARISON	NO LEFT PHASE	LEAD P/P	LAG P/P	LEAD E/P	LAG E/P
Delay Ranking (S, 1 = best)	1	3	2	4	4
Stop Prevention Ranking (S, 1 = best)	1	3	2	4	4
Accidents Ranking (S, 1 = best)	4	3	2 Note E	1	1 Note E
CHARACTERISTIC REQUIREMENTS	NO LEFT PHASE	LEAD P/P	LAG P/P	LEAD E/P	LAG E/P
Change interval required (M)	√	√	√	√	√
Visible yellow change interval (M)	√	√	√	√	√
Yellow trap prevention (S)	√	√	√	√	√
High speed approach (S)	√	No	No	√	√
Multiple left turn lanes (S)	√	No	No	√	√
Three or more opposing lanes (S)	√	No	No	√	√
Insufficient sight distance (S)	√	No	No	√	√
High left turn accident experience (S)	√	No	No	√	√

Notes:

A. Provided the flashing yellow arrow is not used.
D. Where pedestrian signals do not exist.
E. Provided yellow trap has been prevented.
X. These depended on having no clearance interval after the leading green arrow (now prohibited).

Comparisons of Phasings

So far, the material covers mainly intersections with only one left turn phase. But McKay mentions that at least five sequences are available if two left turn phases are used on the same street. In fact, there are seven^#, but two of them are more difficult to implement:

PROPERTIES (R)	SINGLE LEAD	SINGLE LAG	DUAL LEAD	DUAL LAG	DUAL SPLIT LEAD	DUAL SPLIT LAG	NO SPLIT LEAD LAG *	SINGLE SPLIT LEAD LAG	DOUBLE SPLIT LEAD LAG
PHASE DIAGRAMS Phase diagrams at right are for one street. Time flows downward. Smaller diagrams choose one or skip. Orange arrows are flashing yellow arrow turns
P/P w/o yellow trap prevention safe	No	No	Yes	Yes	No	No	n/a	No	No
P/P anti-backup yellow trap prevention safe	Yes	No	Yes	Yes	Yes	No	n/a	No	No
P/P w/ flashing yellow arrows safe	Yes	Yes	Yes	Yes	Yes	Yes	n/a	Yes	Yes
Phase skip causes yellow trap with P/P	Yes	No	No	No	Yes	No	n/a	Yes	Yes
E/P w/o yellow trap prevention safe	No	No	Yes	Yes	Yes	Yes	n/a	Yes	Yes
E/P anti-backup yellow trap prevention safe	Yes	No	Yes	Yes	Yes	Yes	n/a	Yes	Yes
E/P w/ flashing yellow arrows safe	Yes	Yes	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Phase skip causes yellow trap with E/P	Yes	No	No	No	No	No	n/a	No	No
Best at progression platoon meeting point †	Yes	Yes	Yes	Yes	Yes	Yes	No	No	No
Best 1/6 cycle from platoon meeting point †	Yes	Yes	No	No	No	No	Maybe	Yes	Yes
Best 1/3 cycle from platoon meeting point †	Yes	Yes	No	No	No	No	Yes	Maybe	Maybe
Ease of actuation when isolated †	Easy	Mid	Easy	Mid	Easy	Mid	Easy	Mid	Hard
Ease of actuation with progression †	Easy	Mid	Easy	Mid	Easy	Hard	Easy	Mid	Hard
Ease of pretime use †	Easy	Easy	Easy	Easy	Hard	Hard	Easy	Mid	Hard
P/P Unnecessary Delay Ranking (1 = best) †	2	1	9	8	4	3	7	6	5
E/P Unnecessary Delay Ranking (1 = best) †	6	5	8	9	1	3	7	4	2
Use on one way cross street	Yes	Yes †	No	No	No	No	No	No	No
Use where opposing left turns conflict	Yes	Yes †	No	No	No	No	Yes	Yes †	No
Difficulty terminating high speed approach	No	Yes †	No	No	No	Yes †	No	Yes †	Yes
Use if one approach has no left turn lane	Yes	Yes ‡	No	No	No	No	Yes	Yes ‡	No
Use if both approaches have no left turn lanes	Yes	Yes ‡	No	No	No	No	Yes	No	No

* Also known as split phased.
† Installation allowed only if yellow trap has been prevented or warned with a sign.
‡ Yellow trap must be warned with a sign. Caution must be used with the design.
^# There are actually more than seven, but the others have severe disadvantages.
n/a Not applicable, because no permissive turns exist without flashing yellow arrows.

Notes on the items above:

P/P w/o yellow trap prevention safe,
P/P anti-backup yellow trap prevention safe,
P/P w/ flashing yellow arrows safe,
Phase skip causes yellow trap with P/P - Determining whether or not P/P phasings are safe with various levels of yellow trap prevention.
E/P w/o yellow trap prevention safe,
E/P anti-backup yellow trap prevention safe,
E/P w/ flashing yellow arrows safe,
Phase skip causes yellow trap with E/P - Determining whether or not E/P phasings are safe with various levels of yellow trap prevention.
Best at progression platoon meeting point,
Best 1/6 cycle from platoon meeting point,
Best 1/3 cycle from platoon meeting point - Determining the best phasing for various offsets from the ideal platoon meeting points for progression. Some phasings allow (or require) the signal to be farther from the meeting point than other phasings.
Ease of actuation when isolated - A measure of how easy the actuation of the various phasings are to accomplish when the signal is not part of a progression system.
Ease of actuation with progression - A measure of how easy the actuation of the various phasings are to accomplish when the signal is part of a progression system, needing certain times for the beginning and end of the through band.
Ease of pretime use - A measure of how easy the phasings are to accomplish when the signal is part of a progression system, needing certain times for the beginning and end of the through band.
P/P Unnecessary Delay Ranking (1 = best) - This is a ranking of the unnecessary delay caused by only the inefficiencies in the P/P phasing itself, not the overall delay.
E/P Unnecessary Delay Ranking (1 = best) - This is a ranking of the unnecessary delay caused by only the inefficiencies in the E/P phasing itself, not the overall delay.
Use on one way cross street - Determines whether or not the phasing can be used with a one-way cross street.
Use where opposing left turns conflict - Determines whether or not the phasing can be used where the two left turns from the same street interfere with each other, and thus cannot be overlapped in time.
Difficulty terminating high speed approach - Shows whether or not special care must be taken to properly terminate the thru movement of a high speed approach when the timing is mainly being determined by a lagging left turn overlapping that thru movement.

The difference between flashing yellow arrows and the circular green:

Circular green and flashing yellow arrow have the same meaning for drivers who are turning left.
The difference between circular green and flashing yellow arrows is the meaning for drivers who are not turning left.
The circular green permissive phasing causes yellow trap, by releasing the wrong movements simultaneously. If the phase ends before an oncoming similar phase, yellow trap happens. The left turn ends before the oncoming straight ahead ends, trapping a left-turning driver in the intersection, or tricking him into turning across live traffic moving on a green light. With the circular green permissive phasing, left turns and conflicting thru movements are on different phases, which are often on different timing rings, so they can end at different times. This causes yellow trap.
The flashing yellow arrow permissive phasing prevents yellow trap, by releasing the correct movements simultaneously. When the phase ends, the oncoming similar phase has no conflicting movements to interfere with it. Thus, the waiting left turn can turn as soon as the oncoming traffic stops for the light. The left turn ends simultaneously with the oncoming straight ahead, preventing any trapping of left-turning drivers in the intersection, and not tricking drivers into turning across live traffic moving on a green light. With the flashing yellow arrow, all of the movements from one street that conflict with each other are on the same phase, so they must end at the same time.

Properties of Various Phasings:

Left turns allowed on circular greens cause yellow trap if the circular greens end at different times.
Now that we have flashing yellow arrows, there is no excuse for not preventing yellow trap.
Unless flashing yellow arrows are properly used, lagging left turn phases have yellow trap.
Now that flashing yellow arrows prevent yellow trap, there is no excuse for not using lagging left turns.
Left turn phases without left turn lanes cause delays and extra stops.
Flashing yellow arrows cannot be used if left turns and thru traffic share the same lane.
Leading left turns (especially dual split lead) are easiest to actuate. An empty detector changes the signal to the next phase.
Extending a lagging left turn can take the concurrent thru phase beyond the cross street beginning time in the progression plan.
P/P dual split lead utilizes the waste time before both thru phases end, allowing straggler vehicles to make all movements.
P/P dual split lag wastes the time at the end of both turn phases. Only the stragglers turning left can use it.
If phase skips occur, yellow trap can happen with leading left turns. Flashing yellow arrows can prevent such yellow trap events.
On high speed roads, a lagging left can end a concurrent high speed thru movement with vehicles in the dilemma zone.
Lagging P/P phases tend to need less time to empty out than leading ones, provided actuation and yellow trap problems are solved.
The single-split lead-lag phasing is quite useful for progression at intersections that are not at platoon meeting points.
Individual phasing for each signal according to its location in the timing plan is the key to good two-way progression.
Most drivers catch on quickly to mixed lead and lag phasing along a street.

Appendix A: Glossary

TERM	TERM USED HERE	MEANING	DIAGRAM
Advanced green	Leading left turn	A signal sequence where the oncoming thru green is displayed next after the green left turn arrow is displayed. Usually the circular green is shown alongside the green arrow. Often used for the flashing circular green used for this in Ontario.
Circular green permissive	Circular green permissive	Permissive left turns made during a circular green indication. The circular green for a single phase releases the movements shown in the diagram. The red-orange movement must yield to oncoming traffic. This set of movements released together by one indication is a known cause of yellow trap.
Dallas Phasing, or Dallas Display	Dallas Phasing	A signal indication where left turns are made through gaps in oncoming traffic. The Dallas display circular indication is green whenever the oncoming circular indication is green. These indications release the movements shown in the diagram. This phase does not cause yellow trap. Louvers are needed to hide the left turn circular indications from traffic in adjacent lanes. The MUTCD no longer allows this display.
Delayed green	Delayed green	In a single leading left turn, the shortened green received by the oncoming thru phase.
Double-Split Lead-Lag	Double-Split Lead-Lag	Left turn phasing where one direction on the street has a leading left turn, but the opposite direction has a lagging left turn. Either both thru movements or both left turn movements move between the end of the lead phase and the beginning of the lag phase.	See the phasing diagrams above
Dual Lag	Dual Lag	Left turn phasing where both left turns on the street have a protected phase after the thru phase for the same street. The left turns begin and end simultaneously.
Dual Lead	Dual Lead	Left turn phasing where both left turns on the street have a protected phase before the thru phase for the same street. The left turns begin and end simultaneously.
Dual Split Lag	Dual Split Lag	Left turn phasing where both left turns on the street have protected phases after the thru phases for the same street. The left turns can begin at different times, and either or both turns can be skipped.	See the phasing diagrams above
Dual Split Lead	Dual Split Lead	Left turn phasing where both left turns on the street have protected phases before the thru phases for the same street. The left turns can end at different times, and either or both turns can be skipped.	See the phasing diagrams above
Exclusively/Permissive	Exclusively/Permissive	A signal sequence with no left turn phase on the approach. Left turns must filter through gaps in oncoming traffic.
Exclusively/Protected (E/P)	Exclusively/Protected (E/P)	Left turn phasing where the left turn is protected during part of the signal cycle, and is not allowed to proceed during the rest of the signal cycle. The order of words do not indicate a leading or lagging protected phase.
Extended green	Lagging left turn	A signal sequence where the green left turn arrow is displayed next after the oncoming thru green is displayed. Usually the circular green is shown alongside the green arrow. Often used for the flashing circular green used for this in Ontario.
Flashing Yellow Arrow	Flashing Yellow Arrow	A signal indication where left turns are made through gaps in oncoming traffic. The oncoming circular indication is green when the yellow arrow is flashing. These indications release the movements shown in the diagram. This indication does not cause yellow trap.
Flashing Yellow Arrow permissive	Flashing Yellow Arrow permissive	Permissive left turns made during a flashing yellow arrow indication, with an oncoming circular green indication. These indications release the movements shown in the diagram. The red-orange movement must yield to oncoming traffic. This phase is safer than the circular green permissive phase, because it does not cause yellow trap.
Flashing Yellow Ball	Flashing Yellow Ball	A signal indication formerly used in Washington state where left turns are made through gaps in oncoming traffic. The oncoming circular indication is green when the yellow ball is flashing. These indications release the movements shown in the diagram. This phase does not cause yellow trap. The MUTCD no longer allows this display.
Flashing Red Arrow	Flashing Red Arrow	A signal indication where left turns are made through gaps in oncoming traffic after stopping. The oncoming circular indication is green when the red arrow is flashing. These indications release the movements shown in the diagram. This indication does not cause yellow trap.
Flashing Red Ball	Flashing Red Ball	A signal indication formerly used in Michigan, Maryland, and Delaware where left turns are made through gaps in oncoming traffic after stopping. The oncoming circular indication is green when the red ball is flashing. These indications release the movements shown in the diagram. This indication does not cause yellow trap. The MUTCD no longer allows this display.
Lag-Lag	Dual Lag	See dual lag.
Lag-Lag with overlaps	Dual Split Lag	See dual split lag.
Lagging green	Lagging left turn	A signal sequence where the green left turn arrow is displayed next after the oncoming thru green is displayed. Usually the circular green is shown alongside the green arrow.
Lagging left turn	Lagging left turn	A signal sequence where the green left turn arrow is displayed next after the oncoming thru green is displayed.
Lead-Lag	No-Split Lead-lag Single-Split Lead-Lag Double-Split Lead-Lag	Refers to any of the three variations. A phasing where one direction on the street has a leading left turn, but the opposite direction has a lagging left turn. Useful for progression where the intersection is not at a platoon meeting point.
Lead-Lag with overlaps	Double Split Lead-Lag	See double split lead-lag.
Lead-Lead	Dual Lead	See dual lead.
Lead-Lead with overlaps	Dual Split Lead	See dual split lead.
Leading green	Leading left turn	A signal sequence where the oncoming thru green is displayed next after the green left turn arrow is displayed. Usually the circular green is shown alongside the green arrow.
Leading left turn	Leading left turn	A signal sequence where the oncoming thru green is displayed next after the green left turn arrow is displayed.
No-Split Lead-Lag	No-Split Lead-Lag	Left turn phasing where one direction on the street has an exclusive phase, then the opposite direction has an exclusive phase.
Oncoming Traffic, or Opposing Traffic	Oncoming traffic	Traffic coming from the other direction on the same street. Does not represent moving traffic on a different street.
Permissive left turn	Permissive left turn	A signal indication where left turns are allowed to filter through gaps in oncoming traffic.
Permissive-only left turn	Exclusively Permissive	This was rejected because it breaks the rules for the use of the word "only."
Permissive/Protected	Lagging Protected/Permissive (P/P)	Left turn phasing where the left turn is protected during the next part of the signal cycle after it is permissive. The order of words indicates a lagging protected phase.
Phase	Phase	One portion of a traffic signal controller responsible for timing and displaying the indications controlling one or more movements that are always given the right-of-way at the same time.
Phase overlap	Phase overlap	Two phases are said to overlap when they can be given the right-of-way at the same time. They thus overlap in time, but not in space. Each diagram shows two phases overlapping.
Prohibited left turn	Prohibited left turn	A signal indication where left turns are to wait at the stop line until the indication is no longer displayed. The usual indication for this is a red arrow.
Prohibited/Protected	Lagging Exclusively/Protected	Left turn phasing where the left turn is protected during the part of the signal cycle after oncoming traffic is allowed to move. Left turns are not allowed to turn while oncoming traffic is moving. The order of words indicates a lagging protected phase.
Protected left turn	Protected left turn	A signal sequence containing a green left turn arrow, where no other traffic moving in conflict with the turn is given a GO indication. Often right turns on red are allowed to turn in conflict with the protected turn.
Protected-only left turn	Exclusively Protected	This was rejected because it breaks the rules for the use of the word "only."
Protected/Permissive (P/P)	Protected/Permissive (P/P)	Left turn phasing where the left turn is protected during part of the signal cycle, and is permissive during other parts of the signal cycle. In some notations (but not here), the order of words indicates a leading protected phase.
Protected/Prohibited	Exclusively/Protected (E/P)	Left turn phasing where the left turn is protected during part of the signal cycle, and is not allowed to proceed during the rest of the signal cycle. In some notations (but not here), the order of words indicates a leading protected phase.
Separate Phases	No-Split Lead-Lag	Left turn phasing where one direction on the street has an exclusive phase, then the opposite direction has an exclusive phase.
Single lag	Single lag	A signal sequence where the green left turn arrow is displayed in one direction next after the oncoming thru green is displayed.
Single lead	Single lead	A signal sequence where the oncoming thru green is displayed next after the green left turn arrow is displayed to one approach.
Single-Split Lead-Lag	Single-Split Lead-Lag	Left turn phasing where one direction on the street has a leading left turn, but the opposite direction has a lagging left turn. Both thru movements move between the end of the lead phase and the beginning of the lag phase. This is useful for progression where the intersection is not at a platoon meeting point.
Split Phase	No-Split Lead-Lag	Left turn phasing where one direction on the street has an exclusive phase, then the opposite direction has an exclusive phase.
Yellow trap	Yellow trap	A dangerous set of signal indications where left turns are trapped in the intersection with a yellow or red light and flowing oncoming traffic. This is caused by allowing left turns on circular green. If both directions on the same street have circular greens, and then only one direction changes to circular yellow, yellow trap occurs. The left turns receiving the yellow light are trapped. Either drivers are forced to stay in the intersection with a red light, or they try to turn and crash into traffic moving on the remaining green.

Links:

All Yellow Trap Links are here:
YELLOW TRAP INDEX PAGE
Main Yellow-Trap Page
A visual example of flashing yellow arrows.
How Left Turn Signals Work
Left Turn Signals - Then and Now
More on flashing yellow arrow
Unusual Traffic Problems, Hazards, and Solutions
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Leading and Lagging Left Turn Signals Compared