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 labled with even number. Phase 2 and 6 are usually on the main streat. 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 within an additional-file looks like this:
<add>
<tlLogic id="2881" offset="0" 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="0,4,1,2"/>
<param key="ring2" value="0,4,0,6"/>
<param key="barrierPhases" value="4,4"/>
<param key="coordinate-mode" value="true"/>
<param key="coordinatePhases" value="2,6"/>
<param key="offset" value="10" />
<param key="whetherOutputState" value="true"/>
<phase duration="99" minDur="6" maxDur="16" vehext="2" yellow="4" red="1" name="1" state="grrrrrrGGrrr"/>
<phase duration="99" minDur="10" maxDur="67" vehext="2" yellow="4" red="1" name="2" state="grrrrrrrrGGG"/>
<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="grrrrGGrrrrr"/>
</tlLogic>
</add>
<tlLogic> Attributes#
The following attributes/elements are used within the tlLogic element:
| Attribute Name | Value Type | Description |
|---|---|---|
| id | id (string) | The id of the traffic light. This must be an existing traffic light id in the .net.xml file. Typically the id for a traffic light is identical with the junction id. The name may be obtained by right-clicking the red/green bars in front of a controlled intersection. |
| type | NEMA (string) | This value should be kept as NEMA to activate the NEMA-phase controller module |
| programID | id (string) | The id of the traffic light program. |
| offset | int | The offset value here will be overwritten by the param offset below |
<param> Attributes#
The following parameters are used to set the NEMA diagram:
| Parameter Key | Value Type | Description |
|---|---|---|
| detector-length | int | The length (in meters) of the stop bar detectors for non-left-turn lane. Note that the detectors can be replaced by customized detectors. This parameter is for quick generation of a stop bar detector setting. |
| detector-length-leftTurnLane | int | The length (in meters) of the stop bar detectors for left-turn lane. Note that the detectors can be replaced by customized detectors. This parameter is for quick generation of a stop bar detector setting. We have this parameter here because typically the stop bar detectors on the left-turn lanes have different lengths from the through lanes. |
| total-cycle-length | int | The cycle length in seconds. |
| ring1 | string | The phase numbers in ring 1 seperated by comma (','). Fill 0 if a phase does not exist. |
| ring2 | string | The phase numbers in ring 2 seperated by comma (','). Fill 0 if a phase does not exist. Repeat the ring 1 phases of the side of barrier if that side of barrier does not have any phases. |
| barrierPhases | string | One set of phases, seperated by comma (','), that need to end together. This defines a barrier. Usually phase 4 and 8, i.e., "4,8". |
| coordinate-mode | bool (true or false) | Default to false. True if the controller is in coordinated mode. |
| coordinatePhases | string | One set of phases, seperated by comma (','), that need to end together. This defines another barrier. If in coordinated mode, this set of phases are the coordinated phases. Usually phase 2 and 6, i.e., "2,6". |
| offset | double | The offset value in seconds. Default to 0. Only effective when the controller is in coordinated mode. The reference to the offset is always the end of the coordinated phases. |
| whetherOutputState | bool (true or false) | Whether record the signal phase change events. This could be used for generating Automated Traffic Signal Performance Measures (ATSPM). |
| show-detectors | bool (true or false) | It controls whether generated detectors will be visible or hidden in sumo-gui. The default for all traffic lights can be set with option --tls.actuated.show-detectors. It is also possible to toggle this value from within the GUI by right-clicking on a traffic light. |
<phase> Attributes#
Each phase is defined using the following attributes:
| Attribute Name | Value Type | Description |
|---|---|---|
| duration | time (int) | This attribute is default to 99 and does not affect the control. The duration of the phase will change dynamically according to the traffic actuation and constrained by minDur and maxDur. |
| minDur | time (double) | The minimum green of the phase. |
| maxDur | time (double) | The maximum green of the phase. In coordinated mode, it's the split minus the yellow and all red time of the phase. |
| vehExt | time (double) | Vehicle extension in seconds of the actuated phase. |
| yellow | time (double) | The yellow time of the phase. |
| red | time (double) | Red clearance time of the phase. |
| name | int | The NEMA phase number. |
| state | string | The state attribute defines the allowable movements associated with the phase. This is best set from netedit. You can find more information from Traffic Lights -> Signal state definitions |
Change the timings during the simulation#
Certain signal timing parameters can be udpated 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 coordianted 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.