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SAE J2735-Draft-Rev28 [issued: 11-10-08]
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This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Annex D: Traveler Information Message Use and Operation
Traveler Information Introduction
Traveler Information is designed to enable broadcast advisory messages to the vehicle driver based upon
location and situation relevant information. Messages are prioritized both for delivery and presentation
based on the type of the advisory. Presentation to the driver may be in the form of text, graphics, or audio
cues.
Examples include traveler advisories (traffic information, traffic incidents, major events, evacuations, etc.)
and road signs. Traveler advisories are dynamic and temporary in nature. Conversely, road sign messages
emulate their physical counterparts and are static in nature. Differences are discussed in this document.
The message, developed by the SAE-DSRC Traffic and Traveler Information Subcommittee discussed
earlier in this standard, describes the payload of the Traveler Information Message. This Annex describes
when [RS12]the On-Board Equipment (OBU) will receive traveler information as well as how an OBU
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could utilize the data prior to presentation to the driver[RS13].
Traveler Information Message Structure
The following text describes the format of a packet containing multiple individual advisories or road signs.
(Refer to Figure 1: Packet Format, on the following pages)
Packet Structure
Multiple traveler advisories or road signs may be packaged into a single message packet for transmission.
However, it is recommended practice not to mix advisories and road signs within the same packet since
road signs are essentially stable whereas advisories require frequent updates. Additionally, it is advisable
to only couple similar road signs[RS14]. For example, in a specific geographic area, combine all the speed
limit signs into one packet and all the exit services signs into a different packet.
Each packet has a unique Packet ID. If a vehicle’s OBU has processed a packet with a particular ID, it can
then ignore subsequent packets with that same ID, updated packets will have a different ID. The Packet ID
is an octet string which is a combination of an assigned jurisdictional value (in the most significant bytes)
and a locally defined ranges (determined by state and local policies). The locally defined range allows for
assigning further groups of users (data issuers) as well as a unique value for each packet. This process, and
recommended practices, is discussed further in Annex TBD[RS15].
Data Frame Header
All individual message (data frame) headers are of a common format. However, individual messages are
either of type “advisory” or “road sign”. If it is an advisory, the message ID consists of a 2-byte Advisory
Number. This Advisory Number can be used to connect to additional message content transmitted in the
ATIS message format over the TCIP/IP stack, if available. If it is a road sign, the message ID is a
combination of 3D position, direction and an MUTCD code. In addition to a message ID, the header
contains a start time, duration time, and priority. This allows the advisory style of message to be similar to
Dynamic Message Sign (DMS) content or to ATIS event message content (and therefore dynamic),
whereas currently a road sign is typically painted (and therefore static).
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Here OBE is used, elsewhere OBU is used, pick one and stick with it.