• Calibrator
Calibrator

Calibrators#

These trigger-type objects may be specified within an additional-file and allow the dynamic adaption of traffic flows, speeds and vehicle parameters (vTypes). The syntax for such an object is: <calibrator id="<ID>" lane="<LANE_ID>" output="<OUTPUT_FILE>"/\>. They can be used to modify simulation scenario based on induction loop measurements. They can also be used to model location-base changes in driving behavior.

A calibrator will remove vehicles in excess of the specified flow and it will insert new vehicles (of the specified type) if the normal traffic demand of the simulation does not meet the specified number of vehsPerHour. Furthermore, the speed on the edge will be adjusted to the specified speed similar to the workings of a variable speed sign.

Calibrators will also remove vehicles if the traffic on their lane is jammed beyond what would be expected given the specified flow and speed. This ensures that invalid jams do not grow upstream past a calibrator. This behavior can be controlled with attribute 'jamThreshold' (see below).

If no target flow is given, the provided type information will instead be used to modify the type of vehicles that are passing the calibrator.

<additional>
  <vType id="t0" speedDev="0.1" speedFactor="1.2" sigma="0"/>
  <route id="c1" edges="beg middle end rend"/>

  <calibrator id="calibtest_edge" edge="beg" pos="0" output="detector.xml">
    <flow begin="0"    end="1800" route="c1" vehsPerHour="2500" speed="27.8" type="t0" departPos="free" departSpeed="max"/>
    <flow begin="1800" end="3600" route="c1" vehsPerHour="2500" speed="15.0" type="t0" departPos="free" departSpeed="max"/>
  </calibrator>

  <calibrator id="calibtest_lane" edge="middle_1" pos="0" output="detector.xml">
    <flow begin="0"    end="1800" route="c1" vehsPerHour="600" speed="27.8" type="t0" departPos="free" departSpeed="max"/>
    <flow begin="1800" end="3600" route="c1" vehsPerHour="800" speed="15.0" type="t0" departPos="free" departSpeed="max"/>
  </calibrator>

</additional>

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

Attribute Name Value Type Description
id id (string) The id of the calibrator
edge id (string) The id of an edge for measuring and calibrating flow. (Either edge or lane must be specified)
lane id (string) The id of a lane for measuring and calibrating flow (Either edge or lane must be specified)
pos float The position of the calibrator on the specified lane (currently ignored, see [1]
period (alias freq) float The time interval between calibration attempts. default is step-length. Setting a high value limits the maximum achievable flow
routeProbe id (string) The id of the routeProbe element from which to determine the route distribution for generated vehicles.
jamThreshold float A threshold value to detect and clear unexpected jamming if the mean edge speed drops below FLOAT * speedLimit. Range [0, 1]. Default: 0.5 (0.8 in meso)
output file (string) The output file for writing calibrator information or NULL
vTypes string space separated list of vehicle type ids to consider (for counting/removal/type-modification), "" means all; default "".

The flow elements which are defined as children of the calibrator definition follow the general format of flow definitions. As the only difference, either the attribute vehsPerHour or speed or type (or any combination of these) must be given.

By default edge calibrators will use departLane="free" whereas lane calibrators will use departLane="x" where x is the lane index of the calibrator lane. All calibrators default to departSpeed="max".

Decision to insert new vehicles#

The interval (begin, end) defines the time in which calibration takes place. The length of the interval also defines the aggregation period for comparing observed and desired flows. The goal of the calibration is to ensure that the correct number of vehicles are inserted by the end of the interval. At the same time, the space-time structure of existing traffic should be preserved as much as possible.

There is a trade-off between using shorter or longer calibration intervals.

  • With shorter consecutive intervals it may happen that vehicles will be removed first and later inserted because they did not arrive with uniform density
  • With longer intervals it may happen that vehicles are inserted very lated because the calibrator tries to wait for existing vehicles that might still appear

The period attribute defines how often a check for inserting vehicles takes place. This value defaults to the simulation step-length. Larger values conserve computation time but may also lead to a tighter clustering of inserted vehicles.

The algorithm for deciding when exactly to insert (and remove) vehicles is described in Erdmann, Jakob (2012) Online-Kalibrierung einer Mikroskopischen Verkehrssimulation. ViMOS 2012, 29.11.2012, Dresden, Deutschland.

Routes of generated Vehicles#

Whenever the measured flow in a given interval is lower than the specified flow, new vehicles are inserted. If the routeProbe attribute is specified, a route is sampled from the distribution of the named route probe detector. Otherwise the route attribute of the flow is used. Note, that this value may also specify the name of a route distribution.

Calibrating only Flow#

If attribute 'speed' is omitted from the <flow> definition, the calibrator will only affect flow by removing or insertion vehicles:

<additional>
  <vType id="t0" speedDev="0.1" speedFactor="1.2" sigma="0"/>
  <route id="c1" edges="beg middle end rend"/>

  <calibrator id="calibtest_edge" edge="beg" pos="0" output="detector.xml">
    <flow begin="0"    end="1800" route="c1" vehsPerHour="2500" type="t0"/>
    <flow begin="1800" end="3600" route="c1" vehsPerHour="2500" type="t0"/>
  </calibrator> 
</additional>

Calibrating only Speed#

If only attribute 'speed' is given in the <flow> definition, the calibrator acts similar to a variableSpeedSign:

<additional>
  <calibrator id="calibtest_edge" edge="beg" pos="0" output="detector.xml">
    <flow begin="0"    end="1800" speed="10"/>
    <flow begin="1800" end="3600" speed="20"/>
  </calibrator> 
</additional>

Calibrating vehicle types#

When a calibrator flow is defined without attribute vehsPerHour but with attribute type, this defines a type-calibrator. This type of calibrator will modify the types of all passing vehicles (or all vehicles that match the vTypes attribute of the calibrator). The normal behavior is to replace the type of the passing vehicles with the type set in the flow element.

Note

When calibrating types, the 'route' attribute can be omitted from the flow definition

Caution

The type modification happens when the vehicle enters the calibrator edge regardless of the configuration calibrator position.

Type-dependent mapping#

If the traffic consists of multiple vehicle types (i.e. passenger cars and trucks) it may be desirable to either

  • modify only some of the observed types
  • perform a dependent mapping: carType -> carType2, truckType -> truckType2

Both can be accomplished by using the vTypes attribute of the calibrator to make it apply to a subset of types only. For a dependent mapping, multiple calibrators (each with a different vTypes attribute) may be defined. However, if there are very many types this may be cumbersome to define. In this case it may be better to define a mapping between type distributions as described below.

<additional>
  ... 
  <calibrator id="forCars" edge="E1" pos="0" type="myCarType">
    <flow begin="0" end="1800" type="myCarType2"/>    
  </calibrator>
  <calibrator id="forTrucks" edge="E1" pos="0" type="myTruckType">
    <flow begin="0" end="1800" type="myTruckType2"/>    
  </calibrator>  
</additional>

Mapping between vTypeDistributions#

A special behavior is activated if the following conditions are met: - the type in the flow element references a vTypeDistribution - the passing vehicle was defined with a type drawn from a vTypeDistribution - both vTypeDistributions have the same number of member types In this case, the new type of the passing vehicle will be mapped to a specific type in the vType distribution: - the index of the actual vehicle type in the original vTypeDistribution will be computed - the type with that index in the new vTypeDistribution will be used as the new vehicle type

Example route-file input:

    <vTypeDistribution id="dist1">
        <vType id="car" probability="70"/>
        <vType id="truck" maxSpeed="10" probability="30" vClass="truck"/>
    </vTypeDistribution>

Example additional-file input: 

    <vTypeDistribution id="bad_weather">
        <vType id="car2" speedFactor="0.8" decel="3"/>
        <vType id="truck2" decel="2" tau="1.5" vClass="truck"/>
    </vTypeDistribution>

    <calibrator id="c1" lane="middle_0" pos="0" output="detector.xml">
        <flow begin="900"    end="1800" route="r1" type="bad_weather"/>
    </calibrator>`

In this example, all cars will be mapped to slower cars (type 'car' to 'car2') and all trucks will be mapped to trucks that keep larger distances.

Building a scenario without knowledge of routes, based on flow measurements#

Note

A simpler and more robust approach is provided by the routeSampler tool.

Due to their ability of adapting higher as as well as lower flows to a specified value, calibrators may be used to adapt (almost) arbitrary traffic demand to a given set of measurements. One strategy for building a scenario from measurements is therefore, to generated random traffic and use Calibrators in conjunction with route probe detectors.

Each edge where measurements are given should receive a calibrator and a route probe detector. As soon as the first vehicle has passed the route probe detector, the calibrator will be able to use that vehicles route. For the calibrator to be able to function before the first vehicle, it needs a fall back route which just needs to consist of a single edge (i.e. the edge on which the calibrator is placed).

Example additional-file:

<additional>
   <vType id="t0" speedDev="0.1"/>
   <routeProbe id="cali_edge1_probe" edge="edge1" period="60" file="output.xml"/>
   <route id="cali1_fallback" edges="edge1"/>
   <calibrator id="cali_edge1" lane="edge1_0" pos="0" output="detector.xml" period="60" routeProbe="cali_edge1_probe">      
      <flow begin="0"    end="1800" route="cal1_fallback" vehsPerHour="2500" speed="27.8" type="t0" departPos="free" departSpeed="max"/>
      <flow begin="1800" end="3600" route="cal1_fallback" vehsPerHour="2500" speed="15.0" type="t0" departPos="free" departSpeed="max"/>
   </calibrator>
</additional>

Running the simulation with the random demand as well as these <calibrator> and <routeProbe> definitions will achieve a simulation in which traffic matches the specified flows at each calibrator edge. However, the realism of traffic flow behind (or between) calibrators depends on the fit between random routes and real-world routes. The importance of this fit increases with the size and complexity of the network between calibrator edges.