HOW LEFT TURN TRAFFIC SIGNALS WORK

Left turn traffic signals are used to relieve traffic backups in left turn lanes. They provide left turns protected from conflict from oncoming traffic.

A phase is a set of signals controlling a particular traffic movement.

Each left turn phasing is explained in detail here.

All signals shown here are protected against yellow trap with Flashing Yellow Arrows (orange arrows on diagrams).

The various kinds of phasing are determined by the order in which the traffic phases are displayed.

The map symbols shown are used to indicate phasing patterns on maps showing traffic signal locations. These symbols are defined here.

SIMPLE INTERSECTIONS WITHOUT LEFT TURN PHASES
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	No left signals	Permissive							Each street gets one signal phase. Left turns are made through gaps in opposing traffic. See note A.
	Tee, no left signals	Permissive							Each street gets one signal phase. Left turns are made through gaps in opposing traffic. See note A.
	One-way no left signals	Permissive							Each street gets one signal phase. Left turns are made through gaps in opposing traffic. See note A.
The following information pertains to left turn phasing and the diagrams: A phase is one traffic movement or set of movements that all occur together. Permissive (also called exclusively permissive) left turns are made through gaps in opposing traffic. Protected (also called exclusively protected) left turns are made when opposing traffic is stopped. P/P is Protected/Permissive. During one phase, the left turn is protected, and during another phase, they are permissive. A black curved arrow on a diagram indicates a protected turn. An orange dashed curved arrow indicates a permissive turn provided by a flashing yellow arrow (or a circular green, if yellow trap is allowed). For exclusively protected left turn phasing, ignore the orange arrows in the diagrams.
TEE INTERSECTIONS AND ONE-WAY STREETS
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Tee lead	Protected or P/P							The leading left turn is before the opposing thru phase. See note A.
	Tee lag	Protected or P/P							The lagging left turn is after the opposing thru phase. See note A.
	One-way lead	Protected or P/P							The leading left turn is before the opposing thru phase. See note A.
	One-way lag	Protected or P/P							The lagging left turn is after the opposing thru phase. See note A.
In the following sequences, note how a left turn can go with its adjacent thru movement. When two movements go at the same time, it is said that they overlap:
SINGLE LEFT TURN, ONE LEFT TURN INTERVAL
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Single lead	Protected or P/P							The leading left turn is before the opposing thru phase. See notes B and F.
	Single lag	Protected or P/P							The lagging left turn is after the opposing thru phase. See note C and F.
Left turns from opposite directions can also overlap (go simultaneously). The thru movements also overlap:
DUAL LEFT, ONE LEFT TURN INTERVAL
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Dual lead	Protected or P/P							The leading left turns are before the opposing thru phase. The left turns end simultaneously. See notes A and F.
	Dual lag	Protected or P/P							The lagging left turns are after the opposing thru phase. The thru movements end simultaneously. See notes A and F.
The above methods can be combined to produce more efficient sequences. One overlap changes to another overlap when traffic demand ends:
SPLIT LEFT TURNS ON ONE STREET
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Dual split lead	Protected or P/P							The leading left turns are before the opposing thru phases. Note that each left turn phase can end separately. Each one ends as soon as it runs out of traffic. See notes D and F.
	Dual split lag	Protected or P/P							The lagging left turns are after the opposing thru phases. Note that each thru phase can end separately. Each one ends as soon as it runs out of traffic. See notes C and F.
	Dual lead-lag no split	Protected							Each leg of the street with left turn phases has its own phase. Each phase ends as soon as it runs out of traffic. This is also called Split Phasing. See note E.
	Dual lead-lag 1 split	Protected or P/P							The leading left turn is before the opposing thru phase. The lagging left turn is after the opposing thru phase. Note that the leading left turn phase must end before the lagging left turn phase can begin. See note C.
	Dual lead-lag 2 split	protected or P/P							The leading left turn is before the opposing thru phase. The lagging left turn is after the opposing thru phase. Either the leading left turn or its parallel thru phase can end first. Each one ends as soon as it runs out of traffic. See notes C and F.
A set of left turn phases can also be added to each street:
ONE LEFT TURN INTERVAL ON EACH STREET
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Quad lead	Protected or P/P							The leading left turns are before the opposing thru phase. The left turns end together. See notes A and F.
	Quad lag	Protected or P/P							The lagging left turns are after the opposing thru phase. The thru movements end together. See notes A and F.
SPLIT LEFT TURNS ON BOTH STREETS
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Quad split lead	Protected or P/P							The leading left turns are before the opposing thru phases. Note that each left turn phase can end separately. Each one ends as soon as it runs out of traffic. See notes D and F.
	Quad split lag	Protected or P/P							The lagging left turns are after the opposing thru phases. Note that each thru phase can end separately. Each one ends as soon as it runs out of traffic. See note C and F.
	Quad lead-lag no split	Protected							Each leg of each street has its own phase. Each phase ends as soon as it runs out of traffic. This is also called Split Phasing. See note E.
	Quad lead-lag 1 split	Protected or P/P							The leading left turn is before the opposing thru phase. The lagging left turn is after the opposing thru phase. Note that the leading left turn phase must end before the lagging left turn phase can begin. See note C.
	Quad lead-lag 2 split	Protected or P/P							The leading left turn is before the opposing thru phase. The lagging left turn is after the opposing thru phase. Either the leading left turn or its parallel thru phase can end first. Each one ends as soon as it runs out of traffic. See notes C and F.
COMBINED SEQUENCES
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Quad split lead, & lead-lag no split	Protected or P/P							This is a combination of the dual split lead and the dual lead lag, no split. Each street has the characteristics of the dual left turn sequence used. See note D and F.
	Double single lead	Protected or P/P							This is a combination of two single lead left turns. The leading left turns are before the opposing thru phase. See notes B and F.
	Dual split lead & single lead	Protected or P/P							This is a combination of a dual split lead and a single lead. The leading left turns are before the opposing thru phase. See notes B and F.
ROTATING SEQUENCES
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Quad clock- wise no split	Protected							Each leg of each street has its own phase. But the two phases for the same street are not next to each other in time. This is a form of Split Phasing used for progression purposes. See note E.
	Quad counter- clock- wise no split	Protected							Each leg of each street has its own phase. But the two phases for the same street are not next to each other in time. This is a form of Split Phasing used for progression purposes. See note E.
NEW SEQUENCES WITH FLASHING YELLOW ARROWS (FYA)
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Quad lead-lag no split With FYA	P/P							Each leg of each street has its own phase, but with permissive opposing left turns. This is a special version of Split Phasing. See note F.
	Quad clock- wise no split With FYA	P/P							Each leg of each street has its own phase, but with permissive opposing left turns. This is a form of Split Phasing used for progression purposes. See note F.
HALF SIGNALS AND PEDESTRIAN CROSSINGS
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	Half signal	Protected or P/P							The half signal does not stop traffic in one direction. If the west road is two-way, traffic entering from it is restricted to right turns, and moves when the overlapping left turn moves. This signal can always fit into any progression plan.
	Dual half signal	Protected or P/P							There are two completely independent signals at this intersection. Traffic entering from the east-west road is restricted to right turns, and moves when the overlapping left turn moves. This signal system can always fit into any progression plan. See notes A and C.
	Pedestrian signal	Protected							This signal is not at an intersection with another road, but with a pedestrian path. Ignore the right turns in the diagram. See notes A and C.
ODD SEQUENCES FOR PROGRESSION
PHASE ANIMATION	CYCLE SYMBOL & NAME	LEFT SIGNAL TYPE	NORMAL PHASE SEQUENCE   ---- TIME ---->						DISCUSSION
	* Quad separated turns	Protected							The left turns are separated from the opposing thru phase. The two phases for the same street are not next to each other in time. The left turns end together. See note F.
	* Dual double sequence	Protected							The thru phase appears twice in each cycle to accommodate progression or mass transit. See notes A and F.
	* Split double sequence	Protected							The thru phase appears twice in each cycle to accommodate progression or mass transit. See notes A and F.
	* Dual & Split triple sequence	Protected							The thru phase appears 3 times in each cycle to accommodate progression or mass transit. See notes A and F.

* These sequences would be given as a map symbol a circle with a number in it. This would refer to a diagram on one side of the map.

Notes:

1. Beware of yellow trap if pre-emption uses special single direction phases.
2. Beware of yellow trap if phase skip alters the signal sequence. FYA signal must be used on the leg opposing the left turn.
3. Do not use this phasing without FYA or exclusively protected signal.
4. Beware of yellow trap if phase skip alters the signal sequence and FYA not used.
5. This is the only left turn phasing allowed if an approach has a shared lane for left turn and thru movements. FYA not allowed.
6. Do not use if a shared lane for left and thru movements exists on the street.
7. Beware of yellow trap with this signal face.

TIMING RINGS
Traffic signals use what are called timing rings. Each ring contains the timing circuits that time the various intervals needed for several different phases at different times. The phases timed by one ring are phases that can never occur together. Here is how two timing rings can produce phase sequences we see in the above diagrams.
EXCLUSIVELY PROTECTED TIMING RING SETUP TIMING GROUPS Group 1 Group 2 | Group 3 Group 4 | RING 1 Phase 1 Phase 2 | Phase 3 Phase 4 | RING 2 Phase 5 Phase 6 | Phase 7 Phase 8 | CONCURRENCY Group 1 | Group 2 | ------ PHASE ORDER ------>						Each phase must be a member of one ring, one timing group, and one concurrency group. Each ring normally advances from one timing group to the next, in the direction of the phase order arrow. The sequence then repeats. The | marks are the barriers. Both rings must be on the same side of each barrier at the same time. They must cross the barrier simultaneously. The barriers separate the concurrency groups. Phases 1, 2, 5, and 6 are in one concurrency group, and phases 3, 4, 7, and 8 are in the other group. If no traffic is waiting to use a phase, the timing ring can skip it. The ring either will not display any green indication, or it will advance to a later phase. But it must wait to cross the barrier until the other ring has finished its own timing. If no phases in the other concurrency group have waiting traffic, a ring can move backwards within the same concurrency group.
Timing groups in use			Both rings in timing group 1	One ring in group 1, one ring in group 2	Both rings in timing group 2	Both rings in timing group 3	One ring in in group 3, one ring in group 4	Both rings in timing group 4
	Quad split lead	Protected or P/P	Phases 1 & 5	1 & 6 Phases 2 & 5	Phases 2 & 6	Phases 3 & 7	4 & 7 Phases 3 & 8	Phases 4 & 8	The leading left turns occur before the opposing thru phases. Each left turn phase can end separately, as its vehicle queue empties, and its timing ring advances to the next phase. The straight-ahead phases must usually end together, because the rings must cross the barrier together.
To get the other sequences, the various phases are moved to different timing groups or omitted.
CIRCULAR GREEN PROTECTED/PERMISSIVE TIMING RINGS
CIRCULAR GREEN PROTECTED/PERMISSIVE TIMING TIMING GROUPS Group 1 Group 2 | Group 3 Group 4 | RING 1 Phase 1 Phase 2 | Phase 3 Phase 4 | RING 2 Phase 5 Phase 6 | Phase 7 Phase 8 | CONCURRENCY Group 1 | Group 2 | ------ PHASE ORDER ------>						Each phase must be a member of one ring, one timing group, and one concurrency group. Each permissive left turn (orange) and its protected turn are in different rings. Each ring normally advances in the direction of the phase order arrow. Both rings must be on the same side of each barrier | at the same time. The barriers separate the concurrency groups. Phases 1, 2, 5, and 6 are in one concurrency group, and phases 3, 4, 7, and 8 are in the other group. If no traffic is waiting to use a phase, the timing ring can skip it. But both rings must cross the barrier together. If no phases in the other concurrency group have waiting traffic, a ring can move backwards within the same concurrency group. This causes yellow trap. Yellow Trap Example: While phase 6 is active, a change from phase 2 to phase 1 causes yellow trap for the phase 5 movement.
FLASHING YELLOW ARROW PROTECTED/PERMISSIVE TIMING RINGS
FLASHING YELLOW ARROWS P/P TIMING TIMING GROUPS Group 1 Group 2 | Group 3 Group 4 | RING 1 Phase 1 Phase 2 | Phase 3 Phase 4 | RING 2 Phase 5 Phase 6 | Phase 7 Phase 8 | CONCURRENCY Group 1 | Group 2 | ------ PHASE ORDER ------>						Each phase must be a member of one ring, one timing group, and one concurrency group. Each permissive left turn (orange) and its protected turn are in the same ring. Each ring normally advances in the direction of the phase order arrow. Both rings must be on the same side of each barrier | at the same time. The barriers separate the concurrency groups. Phases 1, 2, 5, and 6 are in one concurrency group, and phases 3, 4, 7, and 8 are in the other group. If no traffic is waiting to use a phase, the timing ring can skip it. But both rings must cross the barrier together. If no phases in the other concurrency group have waiting traffic, a ring can move backwards within the same concurrency group. Yellow Trap Prevention: While phase 6 is active, a change from phase 2 to phase 1 prevents yellow trap for the phase 5 movement.

OPTIMUM SIGNAL FACES USED IN ABOVE TABLES
THREE-LIGHT STRAIGHT-AHEAD SIGNAL	FOUR-LIGHT LEFT-TURN SIGNAL FOR FLASHING YELLOW ARROW (FYA) YELLOW-TRAP PREVENTION	SPECIAL THREE-LIGHT FYA SIGNAL FOR OPPOSING LEFT TURNS WITHOUT EXCLUSIVE PHASES	FIVE-LIGHT LEFT-TURN SIGNAL
Standard three-light thru and right signal.	The lower yellow arrow flashes for permissive turns. The upper yellow arrow is for yellow clearance and flashing the entire signal.	This signal is for flashing yellow arrow yellow-trap prevention for opposing permissive left turns (See notes A and B above).	Old standard five-light "doghouse" left-turn signal used with approaches without left turn lane (See note E above).
No sign required.			Requires "LEFT TURN SIGNAL" sign.

OTHER SIGNAL FACES USABLE IN ABOVE TABLES
THREE-LIGHT LEFT-TURN SIGNAL	THREE-LIGHT LEFT-TURN SIGNAL *	FIVE-LIGHT LEFT-TURN SIGNAL	IMPROVED FOUR-LIGHT LEFT-TURN SIGNAL SOLVES PROBLEMS IN PERCEPTION AND STATE LAWS *	SPECIAL FIVE-LIGHT LEFT-TURN SIGNAL FOR FLASHING YELLOW ARROW YELLOW-TRAP PREVENTION *	SPECIAL THREE-LIGHT SIGNAL FOR OPPOSING LEFT TURNS WITHOUT EXCLUSIVE PHASES *
Standard three-light exclusively-protected or Flashing Red Arrows left-turn signal.	Three-light exclusively-protected left-turn signal with circular red.	Standard five-light "doghouse" left-turn signal. See note G.	The yellow arrow flashes for permissive turns. Circular yellow is more recognizable as a clearance indication, and for flashing use. Some states prohibit red arrows.	The left side is for protected turns, the right side is for the flashing yellow permissive turns and flashing the signals. Much easier to wire and understand.	This signal is for flashing yellow arrow yellow-trap prevention for opposing permissive left turns.
No sign required.	Requires "LEFT TURN SIGNAL" sign.       * Does not conform to 2009 MUTCD.

Links:

1. All Yellow Trap Links are here:
   YELLOW TRAP INDEX PAGE
2. Traffic Signal Classification Symbols for maps
3. Left Turn Signals - Then and Now
4. Leading and Lagging Left Turn Signals Compared
5. A visual example of Flashing Yellow Arrows.
6. Unusual Traffic Problems, Hazards, and Solutions
7. HOME