DO THEY THINK WE'RE THAT STUPID?

Is The Flashing Yellow Arrow Being Introduced Because
Drivers Don't Understand The Circular Green?

puzzled person

Various press releases say the purpose of the flashing yellow arrow is to "remove confusion that results from using the circular green for permissive left turns." This has caused the following angry responses from drivers:

5 section signal

But that is not why the flashing yellow arrow was introduced. That's just what some politicians think is the reason.

The real reasons follow:


PART 1


Here's why: Circular green and flashing yellow arrow have very different meanings for traffic that is not  turning left.

This table shows the real differences between the circular green indication and the flashing yellow arrow indication:

CIRCULAR GREEN LEFT TURN
5 section green ball signal
FLASHING YELLOW ARROW
4 section flashing yellow arrow signal
1. Means "go" for all traffic on the approach. 1. Has no meaning for traffic not turning left.
2. Releases other movements beside the left turn. 2. Releases only the permissive left turn.
3. Causes the yellow trap hazard. 3. Prevents the yellow trap hazard.
4. Prevents more efficient signal sequences. 4. Allows more efficient signal sequences.
5. Prevents green progression on most two-way streets. 5. Allows green progression on more two-way streets.
6. Wastes gasoline and driving time. 6. Saves gasoline and driving time.

Circular green is not enough. The advantages of having the flashing yellow arrow are obvious. They are needed in more locations.


PART 2


The following table shows detailed differences between the circular green indication and the flashing yellow arrow indication:

CIRCULAR GREEN LEFT TURN FLASHING YELLOW ARROW
Left turn meaning: permissive turnLeft turn meaning: permissive turn
Straight-ahead meaning: Go Straight-ahead meaning: none (see other signal faces)
Releases other movements.Releases only the left turn movement.
Cannot separate movements.Separates left turn movement from others.
Stops left turns as adjacent thru movement stops.Stops left turns only when the signal controller needs to.
Shorter permissive turn time.
1both left 2westbound green, eastbound red 3circular green
(Permissive turns are shown in orange.)
Longer permissive turn time, saving gasoline.
1both left 2westbound green, eastbound flashing yellow arrow 3green and flashing yellow arrow
(Permissive turns are shown in orange.)
Signal face not allowed over left turn lane.Signal face must be placed over left turn lane.
Shared signal face.Exclusive left turn signal face.
CIRCULAR GREEN LEFT TURN FLASHING YELLOW ARROW
Driver in intersection with yellow light can't turn. Always allows left turns across live oncoming traffic.
Tricks drivers into turning across live traffic.Stops left turns when it is safe to do so.
Causes yellow trap hazard (crash in red)
1both green 2yellow trap 3westbound green, eastbound red
by cutting off left turns before oncoming traffic stops.
Prevents yellow trap hazard
1both green 2no yellow trap 3westbound green, eastbound flashing yellow arrow
by extending left turns until oncoming traffic stops.
Prevents permissive turn with lagging left turn.Allows permissive turn with lagging left.
Permissive turns dangerous with lead-lag sequence.
1eastbound green, westbound red 2both green 3yellow trap
Permissive turns safe with lead-lag sequence.
1eastbound green, westbound flashing yellow arrow 2both green 3westbound green, eastbound flashing yellow arrow
Red arrows stop left turns in lead-lag sequence.
1eastbound green, westbound red 2both thru green 3westbound green. eastbound red
(Permissive turns not allowed)
Flashing yellow arrows allow lead-lag permissive turns.
1eastbound green, westbound flashing yellow arrow 2both green 3westbound green, eastbound flashing yellow arrow
(Permissive turns safely allowed)
Lead-lag sequence discouraged.Lead-lag sequence encouraged.
Progression possible on few two-way streets.Progression possible on more two-way streets.
CIRCULAR GREEN LEFT TURN FLASHING YELLOW ARROW
Worsens two-way progression on many streets.
Cross streets named A - E going horizontal. Block travel time is 1/4 cycle.
Numbers are time (in 1/4 cycles - 2 cycles shown). Time moves downward.
Leading lefts are required for safety. Note widths of green and blue diagonals.
 
1: Aboth green Bboth left Cboth left Dboth green Eboth green
2: Across street Bboth green Cboth green Dcross street Ecross street
3: Aboth left Bboth green Cboth green Dboth left Eboth left
4: Aboth green Bcross street Ccross street Dboth green Eboth green
1: Aboth green Bboth left Cboth left Dboth green Eboth green
2: Across street Bboth green Cboth green Dcross street Ecross street
3: Aboth left Bboth green Cboth green Dboth left Eboth left
4: Aboth green Bcross street Ccross street Dboth green Eboth green
Thru lights are green 1/2 cycle in each direction. Turn lights get 1/4 cycle.
Cross street lights are green 1/4 cycle.
Bandwidth is 1/4 cycle (25%), 50% thru green time - 1/2 flashing yellow arrow.
 Not in a band   Eastbound band   In both bands   Westbound band 

................................................................................................................................

Allows two-way progression on more streets.
Cross streets named A - E going horizontal. Block travel time is 1/4 cycle.
Numbers are time (in 1/4 cycles - 2 cycles shown). Time moves downward.
Lead-lag and flashing yellow arrows. Note widths of green and blue diagonals.
 
1: Aeastbound green, westbound flashing yellow arrow Beastbound green, westbound flashing yellow arrow Cwestbound green, eastbound flashing yellow arrow Dwestbound green, eastbound flashing yellow arrow Eeastbound green, westbound flashing yellow arrow
2: Across street Bboth green Cboth green Dcross street Ecross street
3: Awestbound green, eastbound flashing yellow arrow Bwestbound green, eastbound flashing yellow arrow Ceastbound green, westbound flashing yellow arrow Deastbound green, westbound flashing yellow arrow Ewestbound green, eastbound flashing yellow arrow
4: Aboth green Bcross street Ccross street Dboth green Eboth green
1: Aeastbound green, westbound flashing yellow arrow Beastbound green, westbound flashing yellow arrow Cwestbound green, eastbound flashing yellow arrow Dwestbound green, eastbound flashing yellow arrow Eeastbound green, westbound flashing yellow arrow
2: Across street Bboth green Cboth green Dcross street Ecross street
3: Awestbound green, eastbound flashing yellow arrow Bwestbound green, eastbound flashing yellow arrow Ceastbound green, westbound flashing yellow arrow Deastbound green, westbound flashing yellow arrow Ewestbound green, eastbound flashing yellow arrow
4: Aboth green Bcross street Ccross street Dboth green Eboth green
Thru lights are green 1/2 cycle in each direction. Turn lights get 1/4 cycle.
Cross street lights are green 1/4 cycle.
Bandwidth is 1/2 cycle (50%), 100% thru green time - twice circular green.
 Not in a band   Eastbound band   In both bands   Westbound band 

................................................................................................................................

 B   C   D   E   F   G   H   J     Lack of or less
 B   C   D   E   F   G   H   J     progression
 B   C   D   E   F   G   H   J     means more
 B   C   D   E   F   G   H   J     stops.
 B   C   D   E   F   G   H   J 
 B   C   D   E   F   G   H   J     Note the narrow
 B   C   D   E   F   G   H   J     green and blue
 B   C   D   E   F   G   H   J     diagonals.
 B   C   D   E   F   G   H   J     Progression
 B   C   D   E   F   G   H   J     means fewer
 B   C   D   E   F   G   H   J     stops, saving
 B   C   D   E   F   G   H   J     gasoline.
 B   C   D   E   F   G   H   J 
 B   C   D   E   F   G   H   J     Note the wider
 B   C   D   E   F   G   H   J     green and blue
 B   C   D   E   F   G   H   J     diagonals.
CIRCULAR GREEN LEFT TURN FLASHING YELLOW ARROW
Must be used with single lane approaches.Can't be used with single lane approaches.
Can't be used with wide median left turns.Works fine with wide median left turns.
Can' be used with single lag phasing.Should be used with single lag phasing.
Must be used with shared left/thru lanes.Can't be used with shared left/thru lanes.
Should not be used with dual lane left turns.Should not be used with dual lane left turns.
Dangerous with an offset intersection.Makes offset intersection safe.
Dangerous with some multi-leg intersection legs.Dangerous with some multi-leg intersection legs.
Some pre-emption sequences are dangerous.Properly used makes pre-emptions safe.
Right turn overlap causes yellow trap if not prevented. Right turn overlap causes yellow trap if not prevented.
Can not prevent second yellow trap. Can not prevent second yellow trap by itself.
Can not safely use any of the following:
- lead-lag phasing
- lag-lag phasing
- single lag phasing
Can safely use any of the following:
- lead-lead phasing
- lead-lag phasing
- lag-lag phasing
- single lead phasing
- single lag phasing
- split phasing
Can safely switch a signal face between the following:
- exclusively permissive
- protected/permissive
but not exclusively protected
Can safely switch a signal face between the following:
- exclusively permissive
- protected/permissive
- exclusively protected
Can safely switch a signal between the following:
- lead-lead phasing*
- single lead phasing*
- split phasing

* provided the cross street can not be skipped

Can safely switch a signal between the following:
- lead-lead phasing
- lead-lag phasing
- lag-lag phasing
- single lead phasing
- single lag phasing
- split phasing
Few ways to adjust signal operation to traffic.Many ways to adjust signal operation to traffic.
CIRCULAR GREEN LEFT TURN FLASHING YELLOW ARROW
Wastes driving time with increased stopped time.Saves time by reducing stopped time.
Wastes time with inefficient signal sequences.Saves time with efficient signal sequences.
Wastes driving time with more stops per trip.Saves time with fewer stops per trip.
Wastes driving time through lack of signal progression.Saves time with signal progression.
Wastes gasoline with increased stopped time.Saves gasoline by reducing stopped time.
Wastes gasoline with inefficient signal sequences.Saves gasoline with efficient signal sequences.
Wastes gasoline with more stops per trip.Saves gasoline with fewer stops per trip.
Wastes gasoline through lack of signal progression.Saves gasoline with signal progression.
Wastes time and gasoline.Saves time and gasoline.
Few ways to adjust signal operation to traffic.Many ways to adjust signal operation to traffic.

Flashing yellow arrows are clearly superior to the use of the circular green for permissive turns, provided exclusive left turn lanes exist, and provided they are installed correctly.


PART 3


USAGE

CIRCULAR GREEN LEFT TURN
5 section green ball signal

 
BEST USED ON APPROACHES WITH:
FLASHING YELLOW ARROW
4 section flashing yellow arrow signal

BEST USED ON APPROACHES WITH:
Split phase signal sequences.Exclusive left turn lanes.
Shared left and thru lanes.Left turn lanes without left turn phases.
Only one approach lane.Left turn lanes and offset side street legs.
No exclusive left turn lane.Railroad or emergency vehicle pre-emption.
CIRCULAR GREEN PERMISSIVE
3 section green ball signal

FOR LEFT TURNS ON APPROACHES WITH:
EXCLUSIVELY PROTECTED TURNS
3 section all arrow signal

BEST USED ON APPROACHES WITH:
No left turn phases in both directions.Multiple left turn lanes.
One-way traffic.Three or more oncoming thru lanes.
No oncoming left turn.More than 4 legs at intersection.
Shared left and thru lanes.Sight distance or geometry precluding permissive turns.

Use flashing yellow arrows where they can be used.


PART 4


PHASE SEQUENCES

CIRCULAR GREEN AND ARROW
LEFT TURN
5 section green ball signal
FLASHING YELLOW ARROW
QUAD LEAD
4 section flashing yellow arrow signal
CIRCULAR GREEN
PROTECTED/PERMISSIVE LEAD
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1 north left

Phase 1

south thru right

Phase 2

| east left

Phase 3

west thru right

Phase 4

|
RING 2 south left

Phase 5

north thru right

Phase 6

| west left

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
Causes yellow trap if (for example) phase 1 follows phase 2 due to phase skip. Phase 6 does not end, so the phase 2 permissive turn ends, and is yellow trapped.
FLASHING YELLOW ARROWS
PROTECTED/PERMISSIVE TIMING
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1 north left

Phase 1

south thru right

Phase 2

| east left

Phase 3

west thru right

Phase 4

|
RING 2 south left

Phase 5

north thru right

Phase 6

| west left

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
Prevents yellow trap, even if phases are skipped. The permissive left turn always ends at the same time the oncoming through phase ends.
EXCLUSIVELY PROTECTED
LEFT TURNS
3 section all arrow signal
FLASHING YELLOW ARROW
QUAD LEAD-LAG
4 section flashing yellow arrow signal
EXCLUSIVELY PROTECTED
TIMING RING SETUP
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1 north left

Phase 1

south thru right

Phase 2

| east left

Phase 3

west thru right

Phase 4

|
RING 2 south left

Phase 5

north thru right

Phase 6

| west left

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
Yellow trap does not happen if all left turns are exclusively protected. There is no permissive turn to be trapped.
FLASHING YELLOW ARROWS
PROTECTED/PERMISSIVE LEAD-LAG
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1 south thru right

Phase 2

north left

Phase 1

| west thru right

Phase 4

east left

Phase 3

|
RING 2 south left

Phase 5

north thru right

Phase 6

| west left

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
Phase order does not matter with flashing yellow arrows permissive turns. The permissive turn always ends at the same time as the oncoming through phase.
CIRCULAR GREEN
PERMISSIVE LEFT TURN
3 section green ball signal
FLASHING YELLOW ARROW
SINGLE LAG AND SINGLE LEAD
4 section flashing yellow arrow signal 3 section flashing arrow signal
CIRCULAR GREEN
PERMISSIVE TIMING
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1

Phase 1

south thru right

Phase 2

|

Phase 3

west thru right

Phase 4

|
RING 2

Phase 5

north thru right

Phase 6

|

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
Circular green can be used for left turn when it is impossible for circular greens to end at different times.
FLASHING YELLOW ARROWS
PROTECTED/PERMISSIVE TIMING
TIMING
GROUPS
Group 1 Group 2 | Group 3 Group 4 |
RING 1 south thru right

Phase 2

north left

Phase 1

| east left

Phase 3

west thru right

Phase 4

|
RING 2

Phase 5

north thru right

Phase 6

|

Phase 7

east thru right

Phase 8

|
Concurrency Group 1 | Group 2 |
------ PHASE ORDER ------>
When only one direction has a left turn phase, the 3-section flashing yellow arrows face is used in the opposite direction to prevent yellow trap.



Of course, the circular green is also used for traffic that is not turning left. That's why it causes yellow trap when used for left turns.



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