• 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:

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.