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SAE J2735-Draft-Rev15 [issued: 01-30-07]
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162 -
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.
3.
Probe Data Message Sets
Research has indicated that the optimal DSRC packet size is around 400-500 bytes to account for the effects of
high-speed vehicles. Each Probe Data Message payload is limited to four (4) snapshots in order to accommodate
security and other message overhead in this packet size limitation. If a vehicle contains more than 4 snapshots, it
will send multiple messages. These multiple messages, each containing a maximum of 4 snapshots, make up