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SAE J2735-Draft-Rev18 [issued: 06-26-07]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Annex B Traffic Probe Message Use and Operation
1.
Probe Data
Probe Data is comprised of vehicle attribute and sensor data that is collected and sent from a vehicle OBE
to a local RSE. It is envisioned that this data will be used to ascertain real time road, weather, and traffic
conditions. The post-processed data will be used to advise vehicles approaching the area of current
conditions and suggest appropriate action. This data is collected autonomously as vehicles are traveling
along the roadway system and sent to an RSE when applicable. The message developed by the SAE-DSRC
Traffic and Traveler Information Subcommittee discussed earlier in this standard, describe the payload of
the Probe Data Message. This Annex describes when the OBE should collect Probe Data from the
vehicle’s internal modules/sensors as well as when and where an OBE should send the data.
2.
Probe Snapshots
A Probe Data Message consists of a series of Probe Data Snapshots taken autonomously as the vehicle
travels. These snapshots consist of all Probe Data elements that are available on the vehicle along with the
time and location when each snapshot was taken. It is anticipated that not all vehicles will contain all
Probe Data Elements when the DSRC system is first launched therefore, if a vehicle does not contain a
certain element, it should not send any reference to that element. Facilitated through the use of a variable-
length table of vehicle data elements, this helps keep messages size to a minimum. Each snapshot contains
a “number of vehicle device status fields” field, consisting of one (1) byte, which indicates how many data
elements are contained in the respective snapshot. Each Data Element is comprised of a two (2) byte Type
field and a two (2) Byte Value field. The Type field identifies the element itself (Wipers, Anti-Lock
Brakes, etc) and the Value field contains the data relating to that element for this particular snapshot (10
swipes per minute, Not Active).
As a default, Probe Data Snapshots are taken either based on elapsed time since the last snapshot (i.e.
snapshots are taken every x seconds) or based on an event trigger. For time driven snapshots, the faster the
vehicle is traveling the longer the interval between snapshots. Time Driven Snapshots should be taken
every 20 seconds while the vehicle is traveling at 60 Mph or faster and changes linearly to every 6 seconds
when the vehicle is traveling 20 Mph and slower. The underlying assumptions used to determine these
time intervals are that when the vehicle is traveling 60 mph or faster it is in a rural area where RSE’s are
spaced at 10 mile increments and when the vehicle is traveling 20 mph or slower it is in an urban area
where RSE’s are spaced at 1 mile increments.
A Probe Data Trigger Event is when one or more Probe Data Elements, defined as triggers, change states or
exceeds a specified threshold. For example the Anti-Lock Brake Status element is defined as a Probe Data
Event Trigger. When the Anti-Lock Brake system engages; a Probe Data snapshot is taken.
Vehicle stops and starts are treated as Triggered Events since they could imply a traffic backup and provide
an indication of the travel time of a particular roadway link, among other things. “Start” is defined as when
the vehicles speed exceeds a specified “start” threshold (default is 10 mph) and a “Stop” is defined as when
the vehicle does not move forward for a specified “stop” time (default is 5 seconds) and no other stops have
occurred within the ”last stop” specified time (default is 15 seconds). The “last stop” threshold is intended
to prevent multiple snapshots when vehicles are creeping forward. No snapshots are taken between a
“stop” and “start”.
The OBE should store a minimum of 30 Probe Data Snapshots to ensure data relevancy for areas of
sparsely deployed RSE.