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SAE J2735-Draft-Rev29 [issued: 12-11-08]
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276 -
This is an SAE Motor Vehicle Council draft document of the DSRC committee, subject to change.
Annex F Emergency Vehicle Message Use and Operation
1. Application Description
This is a vehicle to infrastructure message, it is typically sent from an emergency vehicle or transit vehicle.
The Emergency Vehicle Message set consists of three distinct messages, as outlined below. Each will be
discussed in turn in this annex.
Signal Request Message (SRM)
Used to request a preemption or
priority signal state from a signalized
intersection.
Signal Status Message(SSM)
Used to relate the current preemption or
priority signal state(s) a signalized
intersection may be in.
Emergency Vehicle Approaching Message
Used to announce to other vehicles that such
an emergency vehicle is operating in the
local area.
The first two of these messages are used in relation to the control of a signalized intersection during
emergency response operations. The first is transmitted by an emergency vehicle and is used by the
controller of a signalized intersection. The second is output by the local RSU (and originally created by the
signal controller) if a preemption or priority request is granted, causing a change to the signal state status
data of the SPAT message stream being sent, and allows emergency vehicles and priority transit to learn
aspects of the internal state of the controller. The third message is emitted by public safety vehicles while
responding to emergencies, so as to alert other nearby vehicles to that fact.
Restating the signal operations in greater detail: The first message, the Signal Request Message (SRM) is
transmitted by the requesting vehicle (a PSOBU equipped vehicle) to the RSU for that intersection (and
then on to the ASC device). This message is sent after the vehicle has received and processed the “zones”
present in the intersection map message which relates a specific preemption or priority id to a geographical
area. The advanced signal controller (ASC), which is continuously emitting SPAT style messages to relate
the current signal state to other vehicles in the area, will also issue a Signal Status Message (SSM) if there
is one or more active or pending preemption or priority events to report. Note that this message is
transmitted by the local intersection RSU in a broadcast style. There is a potential that multiple local
vehicles will be simultaneously sending Signal Request Messages as they approach the intersection and
receive the RSU/ASC generated Signal Status Message in this time interval. The required logic to decode
the “winner” in such a conflict is outside the scope of this of document and resides in the ASC. The
outcome of that process is reflected in the Signal Status Message. These two messages (along with the
SPAT and MAP message discussed elsewhere) are also considered part of the intersection control message
set.
The third message, or Emergency Vehicle Approaching Message, (EVA) is a simple broadcast message to
alert nearby vehicles of the presence of an emergency vehicle operating in the area. It uses the familiar
ITIS codes for this message, packaged in the format defined in the Roadside Alert message, but adds
additional useful details about the emergency vehicle itself (its use of sirens, lights and other alerts, its basic
type or class of vehicle). The purpose of this message is to provide warning to other drivers when a
PSOBU equipped vehicle such as police, fire or ambulance is in the vicinity and a potential interference
with the emergency vehicle's path is eminent.
Two safety issues are being addressed by the 3rd message. First, the notification to the driver that an
emergency vehicle with its siren/lights flashing is in the area. This can occur even if the emergency lights
are hidden behind an obstruction and the sirens cannot be heard above the surrounding audio (noise, radio,