• SSM Device

Equipping vehicles#

A vehicle can be equipped with an SSM Device which logs the conflicts of the vehicle and other traffic participants (currently only vehicles) and corresponding safety surrogate measures. To attach an SSM device to a vehicle, the standard device-equipment procedures can be applied using <device name>=ssm.

For instance, a single vehicle can be equipped (with a device parametrized by default values) as in the following minimal example

<routes>
    ...
    <vehicle id="v0" route="route0" depart="0">
        <param key="has.ssm.device" value="true"/>
    </vehicle>
    ....
</routes>

Preparing the network file#

The SSM device makes use of the right-of-way information of the intersections to derive some of the measures. It uses the foe matrix of the right-of-way definition to extract crossing and merging paths. If an intersection is set to unregulated, this information is not available and thus e.g. no PET measure can be computed for this intersection.

Output File#

The SSM device generates an output file (one for each vehicle named ssm_<vehicleID>.xml per default, but several vehicles may write to the same file). To set a custom file name, define <param key="device.ssm.file" value="FILE"/> either for the <vehicle> or its <vType> element. Alternatively, set option --device.ssm.file to let all vehicles write into the same file.

The top level elements of the generated file are

<conflict begin="<log-begin-time>" end="<log-end-time>" ego="<equipped-vehicleID>" foe="<opponent-vehicleID>"> ... </conflict>.

Output Content Configuration#

The detail of information given for each conflict and the criteria to qualify an encounter as a conflict (i.e., produce a corresponding conflict element in the output) can be customized by a number of generic parameters to the vehicle or device, resp.. A full parametrization (redundantly assigning the default values, here) could look as follows:

<routes>
    ...
    <vehicle id="v0" route="route0" depart="0">
        <param key="has.ssm.device" value="true"/>
        <param key="device.ssm.measures" value="TTC DRAC PET BR SGAP TGAP PPET MDRAC"/>
        <param key="device.ssm.thresholds" value="3.0 3.0 2.0 0.0 0.2 0.5 2.0 3.4"/>
        <param key="device.ssm.range" value="50.0" />
        <param key="device.ssm.mdrac.prt" value="1.0" />
        <param key="device.ssm.extratime" value="5.0" />
        <param key="device.ssm.file" value="ssm_v0.xml" />
        <param key="device.ssm.trajectories" value="false" />
        <param key="device.ssm.geo" value="false" />
        <param key="device.ssm.write-positions" value="false" />
        <param key="device.ssm.write-lane-positions" value="false" />
        <param key="device.ssm.filter-edges.input-file" value="input_list.txt" />
        <param key="device.ssm.exclude-conflict-types" value="" />
    </vehicle>
    ....
</routes>

Parameters#

The possible parameters are summarized in the following table

Encounter types#

Different types of encounters, e.g. crossing, merging, or lead/follow situations, may imply different calculation procedures for the safety measures. Therefore the SSM-device keeps track of these classifications and provides them in the output to allow the correct interpretation of the corresponding values.

The following table lists the different encounter types along with their codes, which will appear in the output file.

Space Gap Definitions#

• front: position of front bumper of the vehicle as offset from the start of its lane
• back: position of back bumper of the vehicle as offset from the start of its lane

Conflict Types#

Basically, we distinguish between three types of encounters for two vehicles:

• Lead/follow situation: vehicles are passing the same sequence of lanes before and after the conflict point
• Crossing situation: vehicles are passing different sequences of lanes before and after the conflict point
• Merging Situation: vehicles are passing different lanes before the conflict point but the same lane after the conflict point

Conflict Definitions#

For each conflict there is an entry point. For merging and crossing conflicts there is also an exit point. The points may be slightly different for both ego and foe vehicle because the point is computed in reference to the vehicle front or rear bumper whereas a collision could happen with another part of the vehicle.

• entryDist: distance between the front bumper of the vehicle and the entry point along the lane sequence
• exitDist for merging and oncoming encounters: entryDist + vehicleLength
• exitDist for crossing encounters: entryDist + followerLength + leaderWidth
• mergePoint: start of the first common lane of ego and foe vehicle
• crossingPoint: point where the lane geometry sequence (center lines) of both vehicles intersects
• egoCrossingPoint: crossing point shifted upstream along the ego lanes by half of the foe vehicle width
• foeCrossingPoint: crossing point shifted upstream along the foe lanes by half of the ego vehicle width

Available SSMs#

Currently, the following safety surrogate measures are implemented:

• TTC (time to collision)
• DRAC (deceleration rate to avoid a crash)
• PET (post encroachment time)
• MDRAC (modified DRAC)

Further, the following additional safety-relevant output can be generated, which will not be linked to a specific encounter:

• BR (brake rate)
• SGAP (spacing)
• TGAP (time headway)

Please note that some SSMs only apply to a specific encounter or are computed differently for different encounters. For crossing and merging situations, we consider "expected" entry and exit times with respect to the conflict zone. For the calculation of those times for the approaching vehicles, we take into account the current deceleration of the vehicles, if the vehicle is not decelerating, the current speed is extrapolated as a constant (i.e., acceleration is only considered if it is negative).

For some reference to definitions of SSMs see for instance Guido et al. (2011) "Safety performance measures: a comparison between microsimulation and observational data" or Mahmud et al. (2016) "Application of proximal surrogate indicators for safety evaluation: A review of recent developments and research needs".

TTC#

The time-to-collision is defined for all follow-lead situations for which the follower is faster than the leader. It is given as

TTC = space_gap/speed-difference.

For a crossing or merging situation the TTC is only considered defined if for the case that the expected conflict area exit time of the vehicle A is larger than the expected conflict area entry time for vehicle B, where A is the vehicle with the smaller expected conflict area entry time. If this is the case the TTC is defined as

TTC = B’s distance to conflict area entry / B’s current speed.

DRAC#

For a lead/follow-situation the DRAC (deceleration to avoid a crash) where the follower vehicle's speed is larger than the leader vehicle's speed is defined as

DRAC = 0.5*speed_difference^2/space_gap.

For a crossing situation the DRAC is considered defined only if the expected conflict area exit time tA for the first vehicle (A) is larger than the linearly extrapolated conflict area entry time for the second vehicle (B). In that case, the DRAC is defined as follows:

DRAC = 2*(speedB - distConflictB/tA)/tA.

This value is chosen such that constant deceleration with the corresponding rate would imply that B enters the conflict area exactly at time tA, when vehicle A leaves it.

For a merging situation, both variants for the DRAC calculation must be tested and the minimal result should be applied as the appropriate value for the DRAC.

MDRAC#

A modified indicator of the DRAC called MDRAC considering a perception-reaction-time (PRT) is defined as

MDRAC = 0.5*speed_difference/(TTC - PRT).

The PRT is configured with param key device.ssm.mdrac.prt or via option --device.ssm.mdrac.prt and defaults to a value of 1 second.

PET#

For merging and crossing situations, the PET (post encroachment time) is defined as the difference of the leading vehicle's conflict area exit time tA and the following vehicle's conflict area entry time tB:

PET = tB - tA.

For lead/follow situations, no PET is calculated.

BR#

The brake rate is recorded at each simulation step. If the vehicle accelerates, a value of 0.0 is logged.

SGAP#

The spacing is measured as the bumper to bumper distance to the ego vehicle's leader minus the vehicle's minGap

TGAP#

The time headway to the leading vehicle equals spacing/speed.

Output#

The output for an SSM device is written to a file specified by the parameter device.ssm.file in the routes definition file, see above. The extent of output can be controlled by the parameters device.ssm.measures (which SSMs to report) and device.ssm.trajectories (whether to report complete trajectories for vehicles, the tracked SSMs, and the encounter types).

The resulting file contains a root element <SSMLog>, which contains several <conflict> elements and one <globalMeasures> element. Each reported <conflict> corresponds to a tracked encounter, during which an SSM's criticality threshold was exceeded during the simulation. The <globalMeasures> element holds a timeline for all simulated times and the corresponding values for the selected measures, which are not associated to specific conflicts, see Available_SSMs.

An example for the contents of an output file:

<SSMLog>
     <conflict begin="6.50" end="13.90" ego="ego1" foe="foe1">
         <timeSpan values="6.50 6.60 6.70 6.80 6.90 7.00 7.10 ..."/>
         <typeSpan values="10 10 10 10 10 10 10 ..."/>
         <egoPosition values="98.35,61.20 98.35,60.20 98.35,59.25 ..."/>
         <egoLane values="NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 ..."/>
         <egoLanePosition values="59.73 61.12 62.51 63.89 65.28 66.67 68.06 ..."/>
         <egoVelocity values="0.00,-10.23 0.00,-9.78 0.00,-9.33 ..."/>
         <foePosition values="76.31,48.35 77.59,48.35 78.82,48.35 ..."/>
         <foeLane values="WC_0 WC_0 WC_0 WC_0 WC_0 WC_0 WC_0 ..."/>
         <foeLanePosition values="45.84 47.23 48.62 50.00 51.39 52.78 54.17 ..."/>
         <foeVelocity values="13.02,0.00 12.57,0.00 12.12,0.00 ..."/>
         <conflictPoint values="99.23,49.46 99.23,49.46 99.23,49.46 ..."/>
         <TTCSpan values="1.78 1.74 1.70 1.67 1.63 1.60 1.56 ..."/>
         <minTTC time="7.40" position="99.23,49.46" type="10" value="1.48" speed="13.50"/>
         <DRACSpan values="3.66 3.61 3.56 3.50 3.44 3.37 3.30 ..."/>
         <maxDRAC time="6.50" position="99.23,49.46" type="10"
value="3.66" speed="12.95"/>
         <PET time="9.42" position="99.23,49.46" type="17" value="0.72" speed="5.12"/>
     </conflict>
     <conflict begin="21.50" end="27.20" ego="ego1" foe="foe2">
         ...
     </conflict>
     ...
    <globalMeasures ego="ToC_veh">
        <timeSpan values="0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 ..."/>
        <positions values="98.35,61.20 98.35,60.20 98.35,59.25 ..."/>
        <lane values="NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 NC_0 ..."/>
        <lanePosition values="0.00 1.39 2.78 4.17 5.56 6.95 8.33 9.72 11.11 12.50 13.89 ..."/>
        <BRSpan values="0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ..."/>
        <maxBR time="363.20" position="1850.01,-4.95" value="5.53"/>
        <SGAPSpan values="27.50 27.50 27.49 27.45 27.38 27.29 27.17 27.02 26.85 26.65 ..."/>
        <minSGAP time="365.50" position="1863.41,-4.95" value="0.31" leader="leader"/>
        <TGAPSpan values="inf 105.77 52.86 35.19 26.33 20.99 17.41 14.85 12.91 11.39 10.16 ..."/>
        <minTGAP time="365.30" position="1863.14,-4.95" value="0.23" leader="leader"/>
    </globalMeasures>
</SSMLog>

Elements of type <conflict> hold the following information in their child elements:

The <globalMeasures> element has the following structure:

Restricting SSM Device to Edges and Junctions#

The option --device.ssm.filter-edges.input-file <input_list> can be used to specify an input file containing edgeIDs and junctionIDs that are used to restrict the measurement of conflicts. The edges and junctions must be defined as following:

edge:edgeID1
edge:edgeID2
...
junction:junctionID1
junction:junctionID2
...

The edges adjoining the given junctions and the given edges are used to measure the conflicts. Only conflicts occurring at these edges are measured and outputted.

TraCI#

The following values can be retrieved for equipped vehicles if they have enabled the corresponding measures:

• traci.vehicle.getParameter(vehID, "device.ssm.minTTC")
• traci.vehicle.getParameter(vehID, "device.ssm.minPET")
• traci.vehicle.getParameter(vehID, "device.ssm.maxDRAC")

The values correspond to the current min/max for all currently active encounters.

Visualization Examples#

The SSM output file can be used as input to plotXMLAttributes.py. Here is an example: