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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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267 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Figure 12 - Message Recording Loop
While it is recognized that RSU spacing will vary from these assumptions, traffic engineers will have the
ability to actively manage the collection of probe data by changing the snapshot interval parameters for
RSUs in their purview. This management process is defined later.
Event Triggered Snapshots
Event triggered snapshots are triggered by change in vehicle status elements, either a state change (e.g.,
from off to on) or when a value exceeds a specific threshold or undergoes a transition. The purpose of
event triggered snapshots is to gather data on occurrences in the vehicle that are transitory by nature. An
example of an event driven device is traction control switching from off to on. Multiple activations of
traction control at adjacent locations could be used to indicate the location of a slippery road section.
Starts and Stops Snapshots
Snapshots are also generated by stops and starts. Start and Stop events are defined as the following:
A Stop is when there is no movement for a threshold stop time (default stop time threshold = 5
seconds) and no other stops have occurred within another threshold time (default last stop
threshold time = 15 seconds). The latter being intended to prevent multiple counts when cars
creep forward.
A Start is when the vehicle speed exceeds a threshold (default start speed threshold = 10 mph (4.5
m/s)).
As noted previously, no snapshots are taken after a vehicle has experienced a stop event until the vehicle
experiences a subsequent start event.
Starts and stops are useful indicators in a variety of traffic flow measures including incident detection and
clearance and traffic signal operational measures such as cycle failures – where the queue does not
dissipate in the first green phase.
Message Transmission Order
When a vehicle encounters an RSU that advertises the application using a PSID of 5 and a PSC value of 3,
the OBU will send a single Probe Data Message Set, comprised of several individual snapshots, on the
Service Channel indicated by the RSU. Snapshots are sent to the RSU as part of a probe message in the
following order:
1)
Event triggered snapshots are first in the transmission queue from the OBU to the RSU. Since
these often relate to specific adverse conditions that are of interest to traffic operations, these are
considered more critical than the other types of snapshots.
2)
Stops and starts triggered snapshots are second and are needed to provide finer information on
incidents and the various dynamic parameters concerning the traffic flow.
3)
Periodic snapshots are third, oldest first.
The snapshots are queued into groups of four per message, apart from when PSN changes cause a new
message. Messages with start and stop snapshots and/or periodic snapshots should be composed of
snapshots with the same PSN (Probe Segment Number as defined below). Different PSNs should be in
different messages. All of these messages are then sent as a set. Following transmission, all snapshots
including non-transmitted snapshots in the buffer are purged. An implementation of the message transmit
loop is shown below.