• Traffic Lights with NEMA Phases

Traffic Lights with NEMA Phases

The controller modules in Traffic Lights use a stage-based control structure. A phase is defined as a stage of all allowed movements at a time instance. However, traffic signal controllers used in North America widely use National Electrical Manufacturers Association (NEMA) phase definition. A NEMA phase is defined by a certain flow movement at an intersection. At one time, more than one NEMA phase could happen together as long as they do not conflict with each other.

Example: The movements at an intersection are numbered. The right turns usually go with the associated through movements. The left turn movements are usually odd numbers and the through and right turn movements are usually labeled with even number. Phase 2 and 6 are usually on the main street. An intersection does not need to have all 8 phases.

SUMO now includes a controller module that is compatible with NEMA phases. The NEMA-phase controller can perform actuated or coordinated actuated control. This module is developed by Tianxin Li and Qichao Wang at the National Renewable Energy Laboratory and funded by U.S. Department of Energy Vehicle Technology Office.

NEMA Phases#

We can visualize the NEMA phases and timings in Ring-and-Barrier structured NEMA diagrams. For one controller, only one phase from a ring can be activated at a time. Phases from different rings could be activated together as long as they are not from the different sides of a barrier. When a controller is operated in fixed-time control mode, we can model the NEMA phase timing as a corresponding stage-based control timing without any issues. When introducing actuation into the signal control, a Ring-and-Barrier structured traffic signal controller can be more flexible than stage-based controller by allowing different possible phase combinations.

Example: In the above NEMA diagram, we could see phase 1+5, 1+6, 2+6, 3+7, 3+8 and 4+8. However, when the traffic on phase 1 is very low and phase 1 could gap out early. In that case, we will not see phase 1+6 but additionally see phase 2+5.

Defining New NEMA phase timings#

You can load new definitions for NEMA-phase traffic controller as a part of an additional-file. The signal timing could be updated in simulation through TraCI functions. A definition of a traffic light program for a four-leg eight-phase intersection within an additional-file looks like this:

<add>
    <tlLogic id="2881" offset="10" programID="NEMA" type="NEMA">
        <param key="detector-length" value="20"/>
        <param key="detector-length-leftTurnLane" value="10"/>
        <param key="total-cycle-length" value="130"/>
        <param key="ring1" value="1,2,3,4"/>
        <param key="ring2" value="5,6,7,8"/>
        <param key="barrierPhases" value="2,6"/>
        <param key="coordinate-mode" value="true"/>
        <param key="barrier2Phases" value="4,8"/>
        <param key="minRecall" value="2,6"/>
        <param key="maxRecall" value=""/>
        <param key="whetherOutputState" value="true"/>
        <param key="fixForceOff" value="false"/>
        <phase duration="99" minDur="5"  maxDur="25" vehext="2" yellow="3" red="2" name="3" state="rrrrrrrrGrrr"/>
        <phase duration="99" minDur="5"  maxDur="25" vehext="2" yellow="3" red="2" name="7" state="rrGrrrrrrrrr"/>
        <phase duration="99" minDur="5" maxDur="30" vehext="2" yellow="3" red="2" name="4" state="GGrrrrrrrrrr"/>
        <phase duration="99" minDur="5" maxDur="30" vehext="2" yellow="3" red="2" name="8" state="rrrrrrGGrrrr"/>
        <phase duration="99" minDur="5"  maxDur="20" vehext="2" yellow="3" red="2" name="1" state="rrrrrGrrrrrr"/>
        <phase duration="99" minDur="5"  maxDur="20" vehext="2" yellow="3" red="2" name="5" state="rrrrrrrrrrrG"/>
        <phase duration="99" minDur="5"  maxDur="35" vehext="2" yellow="3" red="2" name="2" state="rrrrrrrrrGGr"/>
        <phase duration="99" minDur="5"  maxDur="35" vehext="2" yellow="3" red="2" name="6" state="rrrGGrrrrrrr"/>
    </tlLogic>
</add>

For an intersection at a ramp, the traffic light program configuration could look like this:

<add>
    <tlLogic id="2881" offset="10" programID="NEMA" type="NEMA">
        <param key="detector-length" value="20"/>
        <param key="detector-length-leftTurnLane" value="10"/>
        <param key="total-cycle-length" value="120"/>
        <param key="ring1" value="1,2,0,4"/>
        <param key="ring2" value="0,6,0,4"/>
        <param key="barrierPhases" value="2,6"/>
        <param key="coordinate-mode" value="true"/>
        <param key="barrier2Phases" value="4,4"/>
        <param key="minRecall" value="2,6"/>
        <param key="maxRecall" value=""/>
        <param key="whetherOutputState" value="true"/>
        <param key="fixForceOff" value="false"/>

        <phase duration="99" minDur="6"  maxDur="16" vehext="2" yellow="4" red="1" name="1" state="srrrrrrGGrrr"/>
        <phase duration="99" minDur="10" maxDur="67" vehext="2" yellow="4" red="1" name="2" state="srrrrrrrrGGG"/>
        <phase duration="99" minDur="10" maxDur="22" vehext="2" yellow="3.5" red="1.5" name="4" state="GGGGGrrrrrrr"/>
        <phase duration="99" minDur="10" maxDur="88" vehext="2" yellow="4" red="1" name="6" state="srrrrGGrrrrr"/>
    </tlLogic>
</add>

Example: A ramp intersection which only has phases 1, 2, 4, and 6. Phase 4 controls the traffic from the off ramp.

<tlLogic> Attributes#

The following attributes/elements are used within the tlLogic element:

<param> Attributes#

The following parameters are used to set the NEMA diagram and controller behavior:

<phase> Attributes#

Each phase is defined using the following attributes:

Leading and Lagging Left Turns#

Each ring has four slots: slots 1 and 2 are one side of the barrier, slots 3 and 4 the other. The phase that ends a ring's side at the barrier is the last non-zero phase of that side in the ring order. A left turn placed before the through phase of its ring leads; a left turn placed after it lags. In coordinated mode, a coordinated phase (barrier2Phases) does not need to be last on its side, so every lead/lag arrangement is set by the ring order alone:

When a coordinated phase is followed by a lagging phase:

• If the lagging phase is called, the coordinated phase ends at its force-off and the lagging phase runs next to the coordinated phase of the other ring, ending no later than its own force-off. The lagging phase is served only between the coordinated phase's force-off and its own force-off, and only if its minDur plus the coordinated phase's yellow and red still fit before its force-off. A call that does not fit, or that comes while the coordinated phase is green outside that window (e.g. when the simulation begins elsewhere in the cycle), is served in the next cycle.
• If the lagging phase is not called, the coordinated phase keeps its green until the barrier and so uses the lagging phase's split. This is not green rest.
• If the lagging phase gaps out before the other side of the barrier is called, it keeps its green instead of returning to the coordinated phase, which is past its force-off. It crosses the barrier when the other side is called in either ring and the call still fits (see below); a later call waits for the next cycle. Otherwise the lagging phase ends so that its yellow and red end at the coordinated phase's next start. Meanwhile the coordinated phase of its ring stays red, so with no demand on the other side of the barrier the lagging phase can stay green for the rest of the cycle.
• If the coordinated phase is listed in reservicePhases, a lagging phase that gaps out with no call on the other side of the barrier (in either ring) goes back to it instead, and the coordinated phase is served again in the same cycle. Its lagging phase is not served a second time in that cycle. The coordinated phase times only its minDur then, so a later call on the other side of the barrier can still cross after that minDur, not before the barrier, and if the call still fits. A call that arrives just after the coordinated phase is served again therefore starts up to that minDur later than it would after a lagging phase that keeps its green. If the coordinated phase is still green at its next start, it is timed as usual from there.
• Both rings cross the barrier at the same time, and each ring crosses only if the other side's minDur plus the yellow and red of its own phase still fit before that side's force-off. A call that does not fit for both rings waits for the next cycle, so the coordinated phases start on time.
• If a lagging phase runs to its force-off and its yellow plus red is shorter than that of the coordinated phase in the other ring, the other ring's coordinated phase stays green until the lagging phase ends, and the next side of the barrier starts after the longer clearance. With yellow + red of 4 s on the lagging phase and 7 s on the coordinated phase, the side street starts 3 s later than the splits alone would give. The same holds when the lagging phase is not called: if the yellow plus red of its coordinated phase is shorter than that of the other ring's coordinated phase, the barrier is crossed when its coordinated phase ends and the next side of the barrier starts late by the difference.
• In a lead-lag or lag-lead plan, the coordinated phases do not end together. The coordinated phase in the ring with the leading left turn keeps its green until its own force-off, and it goes back to its leading phase only after that and only while the other side of the barrier is not called; otherwise the leading phase is served after the other side. A coordinated phase always times at least its minDur.
• In a lag-lag plan where both lagging phases are called and both gap out while the other side of the barrier is called, the barrier is crossed when they end, so the other side of the barrier starts early.

The barrier roles come from the ring order in both modes. A warning is written once per controller if barrierPhases names a phase that is not the last on its side (in either mode), or if barrier2Phases does outside coordinated mode. In coordinated mode a coordinated phase followed by a lagging phase is not warned about. These parameters still choose the default phase entered after a barrier crossing.

For controllerType TS2, the cycle zero is the start of the first coordinated phase in the cycle: the coordinated starts are wrapped into the cycle before the earliest one is taken (in the lead-lag row of the table above, the zero is the start of phase 2).

Change the timings during the simulation#

Certain signal timing parameters can be updated during the simulation through TraCI.

• traci.trafficlight.setNemaOffset(tlsID, offset): The offset will be adjusted by shortening or extending the green time of the coordinated phase so that the end of the coordinated phase will match the offset. It is recommended to change the offset gradually to mimic a transition time.
• traci.trafficlight.setNemaMaxGreens(tlsID, maxGreens): sets maximum green times for each NEMA phase by giving 8 numbers. Time 0 must be used for each phase that does not exists
• traci.trafficlight.setNemaSplits(tlsID, splits): works like setNemaMaxGreens but subtracts the red- and yellow-times before setting maximum green time for each phase
• traci.trafficlight.setNemaCycleLength(tlsID, cycleLength): the cycle length may change when you update the splits/max green. You need to set the new cycle length to make the new timing work without problems.

All the updates in the signal timing parameters in the NEMA-phase controller will happen after the current cycle ended.

Configuration Errors#

The following errors may be raised:

• the phases before barrier X from both rings do not add up: EXPLANATION FORTHCOMING
• Ring X does not add to cycle length: EXPLANATION FORTHCOMING

To simulate misconfigured controllers, both errors may be ignored by setting <param key="ignore-errors" value="true"/>.

Publication#

The implementation and validation of the controller was described in Schrader, M.; Wang, Q.; Bittle, J. Extension and Validation of NEMA-Style Dual-Ring Controller in SUMO. SUMO Conf Proc 2022, 3, 1-13.