TRAFFIC SIGNALS: LEFT TURN DEFINITIONS

The following are definitions used for traffic signal design and timing:

Signal indication definitions:

Element definitions:

• Call: A vehicle or pedestrian arriving and actuating a detector, causing the signal to be aware of its presence.
• Conflict: The interference of traffic movements with each other. Two traffic movements conflict if they cross each other's paths, or if they otherwise interfere with each other. Streams of traffic entering the same departure lanes also conflict. Lines of traffic waiting to make one movement conflict with another movement by blocking entry to it.
• Doghouse signal: A shared turn signal with five lights (red, yellow, green, yellow arrow, green arrow) arranged in a cluster, as shown at the right. The name comes from the shape of the signal face, which roughly resembles a doghouse.
• Lagging left turn sequence: A sequence where the oncoming traffic has a circular green just before the left turn traffic gets a green arrow. The oncoming movement ends with a circular yellow, then circular red. Then the left turn traffic is given a green arrow.
• Leading left turn sequence: A sequence where left turns are given a green arrow just before oncoming straight-ahead traffic has a circular green. The left turn phase ends with a yellow arrow, and immediately after it goes dark the oncoming traffic gets a circular green.
• Lead-lag: From one direction, the left turn leads. From the opposite direction, the left turn lags.
• Movement: A single stream of traffic coming from a single direction and leaving in a single direction.
• Overlap: To allow more than one phase to be green at the same time. This is done because the phases involved control traffic movements that do not conflict with each other. An example of overlap is giving opposite straight-ahead movements on the same street green lights at the same time, because they are not conflicting movements.
• Overlap Phase: A special phase unit that has no built-in timing. An overlap phase is associated with, and receives its timing from, two or more parent phases. The parent phases control movements that do not conflict with the movement the overlap phase controls, even though they may conflict with each other. The overlap phase displays a green whenever any of the parent phases displays a green, and shows yellow clearance when the parent phase clears. But the overlap remains green if another parent phase will be the next phase to be green. Overlap phases are commonly used for controlling right turns and flashing yellow arrows.
• Permissive left turn: A traffic signal indication where left turns are made through gaps in oncoming traffic.
• Phase: A single unit that handles assigning right-of-way to one particular movement or set of movements that are always controlled together. It controls one set of signal faces that always display the same color at the same time.
• Pre-emption: A sequence where all approaches but one are given red signals for the purpose of clearing the way for an emergency vehicle, a train, an opening drawbridge, or any other special traffic area required to be quickly emptied of vehicles. It can also keep vehicles from entering an already cleared area during the pre-emption, while moving other traffic.

  Beware of yellow trap during pre-emptions

• Protected left turn: Any traffic signal indication (phase) giving left turns the right to enter the intersection free from conflict with drivers and pedestrians. Usually green and yellow turn arrows are used to indicate this phase.
• Exclusively Protected left turn signal: (also ungrammatically called "protected-only.") A left turn signal face with three lights (red, yellow-arrow, and green-arrow) that stops all left turns when the green arrow is not displayed. Some states use all-arrow heads, others do not allow the red arrow, and use circular red instead.

  Left turns are allowed only when the green arrow is on, not on a circular green displayed on another signal face. The yellow arrow clears out traffic from the green arrow at the end of the phase. The circular red or red arrow stops all left turn traffic until the green arrow shows again.

  The only way yellow-trap can occur at one of these signals is if left turns from the opposite direction are permitted on a circular green.

• Exclusively Permissive left turn signal: (also ungrammatically called "permissive-only.") A signal face with three circular lights (red, yellow, and green) or three arrows (red, yellow, and flashing yellow) that lets left turns filter through gaps in opposing traffic, but gives no protected left turn phase.
• Protected-Permissive (P-P) left turn signal: A left turn signal with five lights (red, yellow, green, yellow arrow, green arrow) that allows left turns to be made through gaps in traffic during the circular green portion of the cycle. A circular red is always used here, because it stops straight ahead as well as left turn traffic.

  Traffic turning left is protected from conflict whenever the green arrow is on. When just the circular green is on, the left turns must yield to oncoming traffic. When the yellow arrow is lit along with the circular green, it means that left turns will no longer be protected from conflict, and will have to yield to oncoming vehicles during the period the circular green is shown alone.

  Yellow-trap can occur with these whenever oncoming traffic is given a left turn arrow immediately after both streets have had circular green.

• Separated phases: Other phases separate two phases for the street on both sides.
• Simultaneous left turns: Both left turn signals act at the same time (opposite of split).
• Split turn phases: The left turn phases can begin or end at different times.
• Split-phase: Each leg of the same road has a separate adjacent phase. This is the same as an unsplit lead-lag.
• Tee intersection: An intersection with three legs. A driveway may not exist where the fourth leg would be.

Special definitions:

• Alternate Flashing Yellow Arrows Display: Prevents Yellow-Trap by using a flashing yellow arrow to extend the opposing permissive left turn through the lag turn phase, without extending the circular green. Flashing yellow arrows needs the special signal face shown at right. This is the same as a protected-permissive signal face, except that an extra yellow arrow is included. The upper yellow arrow and the circular yellow are used for yellow clearance. The lower one flashes for permissive turns. An alternate version of this face has all arrow indications.

  Traffic turning left is protected from conflict whenever the green arrow is on. When the (lower) yellow arrow is flashing, the left turns must yield to oncoming traffic. When the upper yellow arrow is lit, it means that the left turn movement is ending, or that it will have to yield to oncoming vehicles during the period the flashing yellow arrow is shown. The circular yellow means that the flashing yellow arrow period is ending.

  Yellow-trap can not occur if this display is shown to both approaches on the same road. But second yellow trap can occur if not prevented.

  A visual example of flashing yellow arrows.

• Call Redirection: A system where calls that could cause a yellow-trap to occur are redirected, to extend the circular greens instead of changing the signal.
• English turns: Left turns are braided a distance away from the intersection, to drive on the left side of each other.
• False Call Insertion: A system where a left turn call that could cause a yellow-trap to occur also places a cross-street call (or a dummy all-red phase call), to prevent yellow-trap.
• Flashing Yellow Arrows: Prevents Yellow-Trap by using a flashing yellow arrow to extend the opposing permissive left turn through the lag turn phase, without extending the circular green. Flashing yellow arrows needs the special signal face shown at right. This is the same as an exclusively protected signal face, except that an extra yellow arrow is included. The upper yellow arrow is used for yellow clearance. The lower one flashes for permissive turns.

  Traffic turning left is protected from conflict whenever the green arrow is on. When the (lower) yellow arrow is flashing, the left turns must yield to oncoming traffic. When the upper yellow arrow is lit, it means that the left turn movement is ending, or that it will have to yield to oncoming vehicles during the period the flashing yellow arrow is shown.

  Yellow-trap can not occur if this display is correctly implemented and shown to both approaches on the same road.

• Green Trap: This occurs where there are no left turn phases, but the circular greens facing opposite directions can start at different times. This can fool a left turning driver when an opposing driver suddenly gets a circular green and starts to move.

  In green trap, a left turning driver sees an oncoming car slowing down for a red signal, so he thinks he has the right-of-way. As the oncoming driver enters the detector, the signal suddenly turns green, and he speeds up to go straight or right. He enters the intersection in the path of the left turning vehicle. This can cause a crash.

  Green trap happens when opposite circular greens on the same road have their own phase units, but no other phase units are in the concurrency group. The problem happens when one phase stays red until a car appears on the approach.

• Inserted all-red clear: A short period where all of the signals are red between a two-way straight-ahead phase and a lagging left turn phase.

  The only way to prevent second yellow trap is to insert a red period between the permissive right turn and the protected right turn overlapping a cross-street left turn phase.

• Lag-trap: Yellow Trap.
• Left-turn-trap: Yellow Trap.
• Little Yellow Trap: A dangerous situation where the circular greens for both directions end at the same time, but the circular yellows do not. When the shorter yellow turns red, the driver thinks the other direction is red too, and completes his turn. But an oncoming straight-ahead driver could continue on through during this period, causing a crash.

  Little yellow trap happens whenever the yellow clearance intervals of the two circular green phases are set to different values.

• Merge cycle: Extra merging overlap combinations are possible due to channelization.
• Second yellow trap: A dangerous situation that results when a right turn is allowed to continue with a green arrow (to overlap the cross-street left turn phase) while the opposing left turn gets a yellow and red signal. It occurs even if the street the left turn is made from has no left turn phases.
• Shared flashing yellow arrow signal: A signal intended for use where a shared left/thru lane is used with different left turn modes. It can provide up to 6 different indications (red, yellow, green, steady yellow arrow, flashing yellow arrow, and green arrow). The flashing yellow arrow and green arrow share the same lens with a dual color arrow.
• Third yellow trap: A dangerous situation that results when a phase is reserviced in such a way that a flashing yellow or red arrow is terminated before the oncoming thru phase ends. This can come from an oncoming phase or a pedestrian phase that requires the flashing yellow or red arrow to start several seconds after it starts. If the oncoming or pedestrian phase is reserviced, an improper configuration causes the flashing yellow or red arrow to terminate to provide the delay.
• Yellow trap: A dangerous situation that results when a traffic signal controller ends a circular green on one approach before it ends the green on the oncoming approach. It occurs only if left turns are allowed to be made through gaps in oncoming traffic from the approach where the green ends early. This can be due to a left turn phase, or a pre-emption.

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