LEFT TURN SIGNALS
AND
LANE USE CONTROL

MAKING LEFT TURN INDICATIONS MOVE WITH LANE USE

When lane use control (for reversible lanes) was first introduced, the signal displays were easily adapted to varying lane use. Only the circular red, yellow, and green, and the green arrow were in use. But as the MUTCD developed, it prohibited using the circular red for exclusively protected left turns, prohibited darkening a signal over an unused lane, and specified different signal faces for shared and exclusive turn lanes.

For many years, one of the following was done at signalized intersections within lane-use control:

• Signal faces were not moved with the lane assignments.
• The road was widened to accommodate all lane assignments in fixed locations at the intersection.
• Left turns were prohibited or diverted when not in the light traffic mode.
• All signals where left turns were allowed were operated with split phasing (lead-lag with no split).

New developments that will again allow moving indications with lane assignments include:

• The flashing yellow arrow permissive turn indication
• Interpretation 4(09)-15 (I) allows a 5-section signal face to have 6 indications with a dual color arrow section for variable left turn modes with a shared thru/left lane
• Traffic signal controllers can be programmed to change phase sequence on demand.
• Solid state relays allowing the change of detector and signal face assignments without reprogramming the signal controller.

Only one more development is needed to make this possible: a triple aspect red signal. It is not essential, but makes assigning signal faces much easier.

There are three versions of this signal lens:

• The all-red version in the top row
• Two versions of a colorblind-safe signal in the other two rows (makes red look different from yellow)

The lens has three aspects:

• The circular red for straight ahead or shared use (left column)
• The red arrow for an exclusive turn lane (center column)
• The red X when the lane is not to be used (right column)

With LED technology, this special red section should be easy to make. It makes the following lane-use control system possible:

TABLE OF LANE ASSIGNMENT

KEY:

• Opposing Signal: The opposing signals are signals facing oncoming traffic in front of the motorist, presented upside down, so the lanes line up.
• The signals are shown in various signal phasings that are possible. All possible phasings are listed for each case.
• MUTCD = Manual Of Uniform Traffic Control Devices for Streets and Roads, US Department of Transportation
• Yellow Trap is prevented in all of these implementations (more)
• E/P = Exclusively protected left turn (left turns only on green arrow)
• P/P = Protected/Permissive left turn (left turns on green arrow or flashing yellow arrow)
• P = Permissive left turn (left turns only on flashing yellow arrow)
• For traffic unbalanced in the other direction, trade the oncoming display with the motorist's display.
• Except for the flashing speed of the arrows, the animation is running about double to triple the normal speed.

Opposing Signals (upside down)	Signal Display as seen by motorist	Conditions	Phasings Allowed	Comments
		1. BALANCED FLOW 2 lanes each direction Center lane left turn Lead-Lag (single-split P/P)	Single Lead E/P, P/P, with P Single Lag E/P, P/P, with P Dual Lead E/P, P/P Dual Lag E/P, P/P Dual Split Lead E/P, P/P Dual Split Lag E/P, P/P Split phase (Unsplit Lead-Lag) E/P, P/P Single-split Lead-Lag E/P, P/P Double-split Lead-Lag E/P, P/P Separated Phases E/P, P/P
		2. BALANCED FLOW 2 thru lanes each direction Center lane left turn Dual Split Lead P/P	Single Lead E/P, P/P, with P Single Lag E/P, P/P, with P Dual Lead E/P, P/P Dual Lag E/P, P/P Dual Split Lead E/P, P/P Dual Split Lag E/P, P/P Split phase (Unsplit Lead-Lag) E/P, P/P Single-split Lead-Lag E/P, P/P Double-split Lead-Lag E/P, P/P Separated Phases E/P, P/P
		3. UNBALANCED 3 TO 2 3 thru lanes heavy way, 2 thru lanes other way Center lane in heavy direction Each direction's leftmost lane shared thru/left Split phase (Unsplit Lead-Lag) E/P	Single Lead E/P, P/P, with P Single Lag E/P, P/P, with P Split phase (Unsplit Lead-Lag) E/P, P/P Single-split Lead-Lag P/P Separated Phases E/P, P/P	Note that a permissive left turn across three or more lanes of oncoming traffic is not recommended.
		4. UNBALANCED 3 TO 2 3 thru lanes heavy way, 2 thru lanes other way Center lane in heavy direction Each direction's leftmost lane shared thru/left Lead-Lag (single-split P/P)	Single Lead E/P, P/P, with P Single Lag E/P, P/P, with P Split phase (Unsplit Lead-Lag) E/P, P/P Single-split Lead-Lag P/P Separated Phases E/P, P/P	Note that a permissive left turn across three or more lanes of oncoming traffic is not recommended.
		5. UNBALANCED 2 TO 1 2 thru lanes heavy way, 1 thru lanes other way Two lane left turn Lead-Lag (single-split E/P)	Dual Lead E/P Dual Lag E/P Dual Split Lead E/P Dual Split Lag E/P Split phase (Unsplit Lead-Lag) E/P Single-split Lead-Lag E/P Double-split Lead-Lag E/P Separated Phases E/P	Note that a permissive left turn from more than one lane is not recommended. A second signal face of thru indications is needed to meet MUTCD requirements. It would be to the right of the rightmost signal face shown, as seen by oncoming traffic.
		6. UNBALANCED 3 TO 1 3 thru lanes heavy way, 1 thru lane other way One lane left turn Lead-Lag (single-split P/P, E/P)	Single Lead E/P, P/P, with P Single Lag E/P, P/P, with P Dual Lead E/P, P/P Dual Lag E/P, P/P Dual Split Lead E/P, P/P Dual Split Lag E/P, P/P Split phase (Unsplit Lead-Lag) E/P, P/P Single-split Lead-Lag E/P, P/P Double-split Lead-Lag E/P, P/P Separated Phases E/P, P/P	Because a permissive left turn across three lanes is not recommended, the oncoming approach has no permissive left turn. A second signal face of thru indications is needed to meet MUTCD requirements. It would be to the right of the rightmost signal face shown, as seen by oncoming traffic.
		7. UNBALANCED 4 to 1 4 thru lanes heavy way, 1 thru lane other way Each direction leftmost lane shared thru and left Split phase (Unsplit Lead-Lag E/P)	Split phase (Unsplit Lead-Lag) E/P, P/P Separated Phases E/P, P/P	Note that a permissive left turn across three or more lanes of oncoming traffic is not recommended. A second signal face of thru indications is needed to meet MUTCD requirements. It would be to the right of the rightmost signal face shown, as seen by oncoming traffic.

Signal Operation and Lane Use Change

When the lane use is changed, the signal indications must change with the lane signals in a way to preserve safety.

• There are three kinds of lane use changes:
  1. A full change moves all of the lane functions over one lane.
  2. A half change from exclusive turn lanes changes a lane reserved for left turns to a shared left and thru lane in one direction. The leftmost thru lane in the other direction becomes a shared left and thru lane. Left turns might be prohibited or diverted, rather than using shared lanes.
  3. A half change to exclusive turn lanes changes a shared left and thru lane to a lane reserved for left turns in one direction. In the other direction, the shared lane becomes a thru lane, and a new left turn lane opens.
• The lane use must change by only one lane at a time at most.
• A lane use assignment is a specific combination of lane uses.
• A signal face assignment is the set of traffic signal displays for a given lane use assignment.
• The configuration phasing is a chosen phasing compatible with the current lane-use indication.
• The old configuration phasing is the phasing chosen for the old lane-use selection.
• The new configuration phasing is the phasing chosen for the new lane-use selection.
• The transformation phasing is a phasing that is safe during any portion of the change of lane use.
• The safe changing period is when the traffic signal is changing from the lane-controlled street to the cross street, while all of the traffic signals for the intersection are red.
• An advance lane-use signal is the lane-use signal traffic comes to just before a traffic signal.

The following sequence should be used when changing lane use. For each traffic signal separately, at the next safe changing period, do the following: If necessary, change the phasing to the intersection's transformation phasing. Wait at least 4 minutes after all signals have changed phasing before continuing. If a lane is being closed to traffic in one direction, do the following: Leave advanced lane-use signals displaying left-turn indications unchanged. Change all other lane-use signals for the closing lanes to yellow X. Wait at least 4 minutes before continuing. If any lanes have yellow X indications, do the following on approaches losing lanes: For each traffic signal separately, at the next safe changing period, do all of the following simultaneously: Change the advanced left-turn lane-use signal to yellow X. Change signal faces for those lanes to the new signal face assignments. After the last change happens, wait the designed yellow X lane clearance period. Do the following: Change all yellow X indications to red X. Change down green arrows to white left-turn arrows where needed. Change signal faces for those lanes to the new signal face assignments. Wait the designed red-X lane clearance period. For each traffic signal separately, at the next safe changing period, do all of the following simultaneously: Change all lane-use signals to the new configuration. Change all signal faces to the new signal face assignment. Wait at least 4 minutes after the last change of signal face assignment. For each traffic signal separately, at the next safe changing period, do all of the following: If necessary, Change the signal phasing to the new configuration phasing. The important part of this is to not send any vehicles in a lane into signals that are not ready for them.

Animation Key: Red clearances are not shown. All times are speeded up except flashing yellow arrow flash rates. Only one signal cycle is shown between changes, constituting the delay periods mentioned. The numbers during the all red periods correspond to the numbered list at left. B means using or heading to balanced flow. U means using or heading to unbalanced flow.

If the Triple-Aspect Red Section is not Available

What can be done before the FHWA allows a multiple aspect red section? The problem is twofold:

The difference between the circular red and the red arrow:

• If a single section can't display both aspects, interpretation 4(09)-15 (I) allows the use of just the circular red.

The difference between the other red indications and the red X

• There are several other ways to get around this problem:
  1. Repeat the leftmost indication farther left when necessary. The MUTCD allows this.
  2. Repeat the leftmost indication farther left when necessary with limited visibility faces. MUTCD allows this.
  3. Repeat the circular indication far left when necessary. MUTCD allows this.
  4. Display red signals over the closed lanes. This would require a special exception.
  5. Use some device to turn away or hide the signal faces that are dark.
  6. Automatically turn and reconnect the face to control traffic from the opposite direction.
  7. Place a lane control signal above the traffic signal face, and darken the face when the red X is on.

NOTES

• Flashing yellow arrows are used for all permissive movements to prevent yellow trap.
• It is not a good idea to have a permissive turn across three or more lanes of oncoming traffic.
• Although it has been done, it is not a good idea to have a permissive turn from more than one lane.
• If left turning drivers in a shared lane try to pass stopped thru vehicles (e.g. case 4) when a flashing yellow arrow is shown with a circular red, the phasing should be changed to split phase. A DO NOT PASS sign might help.
• An additional indication of a shared turn/thru lane is needed. The changeable sign at right would help.
• The MUTCD requires two thru signal faces. The animations do not show the extra face to the right of the rightmost lane.
• The double left turn arrows indication can be used between signalized intersections, but not just before signals. The single left turn indication must be used over that lane approaching the signal and the red X must be used for that lane leaving the signal.

Links:

• All Yellow Trap Links are here:
  YELLOW TRAP INDEX PAGE
• How Left Turn Signals Work
• Leading and Lagging Left Turn Signals Compared
• A visual example of Flashing Yellow Arrows
• DO YOU KNOW WHAT YELLOW-TRAP IS?
• Unusual Traffic Problems and Solutions
• Home