These trigger-type objects may be specified within an additional-file and allow the dynamic adaption of traffic flows and speeds. 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. A calibrator will remove vehicles in excess of the specified flow and it will insert new vehicles 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 jammend beyond what would be expected given the specified flow and speed. This ensures that invalid jams do not grow upstream past a 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 |
|freq||float||The aggregation interval in which to calibrate the flows. default is step-length|
|routeProbe||float||The id of the routeProbe element from which to determine the route distribution for generated vehicles.|
|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), "" 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 both) 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 freq 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 tigher 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.
Building a scenario without knowledge of routes, based on flow measurements
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. While this can in principle be done with DFROUTER as well, the method described here is more robust for highly meshed networks as found in cities.
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).
<additional> <vType id="t0" speedDev="0.1"/> <routeProbe id="cali_edge1_probe" edge="edge1" freq="60" file="output.xml"/> <calibrator id="cali_edge1" lane="edge1_0" pos="0" output="detector.xml" freq="60" routeProbe="cali_edge1_probe"> <route id="cali1_fallback" edges="edge1"/> <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.