LEFT TURN SIGNAL DESIGN TABLE

Each type of left turn signal is evaluated here to determine whether it is safe from yellow-trap (lag-trap):

KEY COLOR	HAZARD LEVEL
AQUA	Yellow-trap hazard can't ever happen with special signals below
GREEN	No yellow-trap hazard
YELLOW	Hazard exists during phase skip or other unusual condition
RED	Hazard happens every cycle, or very frequently

SIMPLE INTERSECTIONS
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	No lefts	(none)	SAFE
	Tee lead	Either 3 or 5 light	SAFE
	Tee lag	Either 3 or 5 light	SAFE
	One-way lead	Either 3 or 5 light	SAFE
	One-way lag	Either 3 or 5 light	SAFE

SINGLE LEAD LEFT, ONE LEFT TURN INTERVAL
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Single lead	Either 3 or 5 light	Phase skip hazard			Safe without skip:
	Single lead with all-red clear	Either 3 or 5 light	SAFE			Flashing yellow arrow	(see Single lag, reverse the phase sequence)

SINGLE LAG LEFT, ONE LEFT TURN INTERVAL
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Single lag	Either 3 or 5 light	ALWAYS DANGER			Prohibit opposite direction left
	Single lag with all-red clear	Either 3 or 5 light	SAFE
	Single lag /w flashing yellow arrow	special 4 or 5 light, opposing special 3-light	SAFE

DUAL LEFT, ONE LEFT TURN INTERVAL
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Dual lead	Either 3 or 5 light	SAFE
	Dual lag	Either 3 or 5 light	SAFE

SPLIT LEAD LEFT TURNS ON ONE STREET
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Dual split lead	3 light	SAFE
	Dual split lead	5 light	Phase skip hazard			Flashing yellow arrows	(see Quad split lead)
	Dual split lead with all-red clear	5 light	SAFE

SPLIT LAG LEFT TURNS ON ONE STREET
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Dual split lag	3 light	SAFE (but rather useless)
	Dual split lag	5 light	ALWAYS DANGER			flashing yellow arrows

LEAD-LAG ON ONE STREET
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Dual lead-lag no split	(5 light)	SAFE		Other name: Split- phase
	Dual lead-lag 1 split	3 light	SAFE
	Dual lead-lag 2 split	3 light	SAFE
	Dual lead-lag 1 or 2 split	5 light	ALWAYS DANGER			flashing yellow arrows	(see Quad split lead-lag)

ONE LEFT TURN INTERVAL ON EACH STREET
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad lead	3 light	SAFE
	Quad lead	5 light	SAFE
	Quad lag	Either 3 or 5 light	SAFE

SPLIT LEAD LEFT TURNS ON BOTH STREETS
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad split lead	3 light	SAFE
	Quad split lead	5 light	Phase skip hazard			Detector redirection prevents backward skip

SPLIT LEAD LEFT TURNS ON BOTH STREETS - FIXES
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad split lead /w flashing yellow arrows	special 4 or 5 light	SAFE
	Quad split lead with red clear	5 light	SAFE

SPLIT LAG LEFT TURNS ON BOTH STREETS
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad split lag	3 light	SAFE (but rather useless)
	Quad split lag	5 light	ALWAYS DANGER			flashing yellow arrows	(see Quad split lead, reverse the phase sequence)

LEAD-LAG ON BOTH STREETS
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad lead-lag no split	(5 light)	SAFE			Other name: Split- phase
	Quad lead-lag 1 split	3 light	SAFE
	Quad lead-lag 2 split	3 light	SAFE
	Quad lead-lag 1 or 2 split	5 light	ALWAYS DANGER			flashing yellow arrows

MIXED AND OTHER SEQUENCES
PHASE ANIMATION	PHASE CYCLE NAME	LEFT SIGNAL TYPE	YELLOW- TRAP HAZARD	NORMAL PHASE SEQUENCE ------>		CURE?	CURE ANIMATION
	Quad split lead lead-lag no split	3 light	SAFE
	Quad clock- wise no split	3 light	SAFE

THREE-LIGHT LEFT-TURN SIGNAL	FIVE-LIGHT LEFT-TURN SIGNAL	FOUR-LIGHT LEFT-TURN SIGNAL FOR FLASHING YELLOW ARROW YELLOW-TRAP PREVENTION	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 left-turn signal.	Standard five-light "doghouse" left-turn signal.	The lower yellow arrow flashes for permissive turns. The upper yellow arrow is for yellow clearance and flashing the entire signal.	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.
Requires "LEFT TURN SIGNAL" sign.		No sign required.	Requires "LEFT TURN SIGNAL" sign.

SIGNAL GLOSSARY
TERM	DEFINITION
YELLOW-TRAP	Trapping left turning drivers in the intersection with a red light, while opposing traffic still has a green.
LAG-TRAP	Yellow-Trap.
LAG-LEFT-TURN-TRAP	Yellow-Trap
TEE	Tee intersection. A driveway may not exist where the fourth leg would be.
LEAD	Left turn goes immediately before oncoming straight ahead.
LAG	Left turn goes immediately after oncoming straight ahead.
LEAD-LAG	From one direction, the left turn leads. From the other direction, the left turn lags.
SEPARATED	Other phases separate the two phases for the street on both sides.
SIMULTANEOUS	Both left turn signals act at the same time (opposite of split).
SPLIT	The left turn phases can begin or end at different times.
SPLIT-PHASE	Each leg of the road has a separate adjacent phase. This is the same as an unsplit lead-lag.
ENGLISH	Left turns are braided a distance away from the intersection, to drive on the left side.
MERGE	Extra merging overlap combinations are possible due to channelization.
FLASHING YELLOW ARROW PHASE	Prevents Yellow-Trap by extending the opposing permissive left turn through the lag turn phase.
Where both roads have left turn phases, the phasing of each road should be analyzed as a single or dual left turn from the table above. But if an English or merge system is used, the entire phasing pattern must be analyzed instead.
If a special sequence exists for unusual geometry, special pedestrian movements, or pre-emptions for emergency vehicles, railroads, drawbridges, or other unusual conditions, that sequence must be checked for the yellow-trap hazard if: Left turns are permitted on circular greens, and The circular greens can terminate separately.

Links:

All Yellow Trap Links are here:
YELLOW TRAP INDEX PAGE
Unusual Traffic Problems, Hazards, and Solutions
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