Frequently Asked Questions
How do you set up the system?
The Priority Command vehicle transmitter installs like any mobile radio.
It has three cable connections: two for the antennas, and one
for power, ground, and the lightbar and turn signals. It has
one DIP switch inside the case to set the vehicle's priority
level. It requires no other calibration or setup. Once it is
installed, and the antennas are mounted and connected, it's ready
The Priority Command vehicle transmitter can be installed in, on, or
under the dash, or in the center console. Additionally, if the
driver does not need access to the Override On/Off or 4-way-red
switches, it can be mounted behind the dash, under the seat,
or even in the trunk.
The Priority Command intersection receiver requires only a few quick
steps to configure. Just set the DIP switches inside the case
according to a simple chart
for the intersection size, speed limit, and yellow light time,
and to enable or disable around-the-corner, 4-way-red, and low-priority
The Priority Command cabinet-mount intersection receiver can be installed
inside the traffic cabinet using screws or adhesive. It has two
cable connectors: one for the external antenna, and one that
connects to power, ground, and the preemption inputs on the intersection
signal controller. A 12 Vdc wall cube adapter is included.
The receiver must then store
the GPS coordinates for the center of the intersection. This
is accomplished with a few simple steps:
1. Push the button on the receiver
case and hold it in for two seconds. When the LED starts to flash,
release the button.
2. Position a transmitter-equipped
vehicle in the center of the intersection.
3. Key the transmission from
the vehicle using the Override On switch on the transmitter.
The transmit status indicator light on the transmitter will begin
to flash green.
Once the receiver has saved the
GPS coordinates from the vehicle, the LED on the receiver will
blink quickly 10 times and return to solid green indicating that
the coordinates for the center of the intersection have been
saved in nonvolatile memory. The receiver then returns itself
to normal operating mode.
4. End the transmission from
the vehicle using the Override Off switch on the transmitter.
The vehicle may now leave the intersection.
Must the receiver be connected to
a computer signal controller inside a traffic cabinet?
Not necessarily. The receiver's
output relays close whenever a preemption is called for. While
these outputs are normally connected to the preemption inputs
on a computer signal controller, they can also be used to control
dedicated warning lights at uncontrolled intersections, driveways,
crosswalks, or anywhere else they are needed. Additionally, the
Priority Command RX-1A all-weather outdoor receiver does
not need to be installed inside a traffic cabinet and can even
be powered by solar-charged batteries.
Is the system compatible with all
The Priority Command system is designed to work with all NEMA and 170
(CalTrans) controllers that support preemption. While it may
be possible to adapt the system to older electromechanical controllers,
this is not recommended or supported.
Will this system disrupt traffic
Any emergency vehicle on an emergency
run will disrupt traffic flow, with or without preemption. When
an emergency vehicle has to cross an intersection against a red
light, it must slow down or even stop for several seconds to
make sure all lanes of cross traffic will yield before proceeding.
However, if the light is green, the vehicle can cross the intersection
with minimal disruption to traffic flow.
Preemption systems can create
significant disruption to traffic flow if the preemption starts
too soon or remains in effect too long. The Priority Command system is programmed to change the lights
at an appropriate distance to give the emergency vehicle a comfortable
margin, and to end the preemption as soon as the vehicle reaches
the intersection, thereby minimizing the disruption of normal
How does the system work?
The Priority Command vehicle transmitter starts transmitting the vehicle's
GPS coordinates whenever its lightbar is in use or when the driver
has pushed the Override On button. The Priority Command intersection receiver tracks the location, speed,
and heading of the transmitting vehicle when it comes within
its range. The receiver uses a number of factorsintersection
size, yellow light time, roadway speed limit, and vehicle speedto
calculate the appropriate preemption distance. When the vehicle
reaches that distance, the receiver issues a preemption command
to the traffic signal controller for the appropriate direction.
Does the system do event logging?
No, the Priority Command system does not store data from preemption events.
However, good records of these events may be obtained from dashcams
and/or intersection surveillance cameras.
How does it work for odd-shaped intersections
or intersections that are turned at an angle?
Priority Command intersection receivers can be custom
programmed for nearly any intersection. You will need to provide
a blueprint for the intersection that is drawn to scale and accurately
dimensioned with angles referenced to true north. The drawing
should be scaled to include the approach routes for 1000 feet
in all directions. Your Priority
can quote an exact price and delivery schedule for custom intersection
What about elevated freeways or overpasses?
With custom programming, the
system can use GPS altitude data to discriminate between vehicles
at different elevations. It can, for instance, give a preemption
to a vehicle on a surface street while ignoring another vehicle
on an elevated roadway. Contact your distributor for details.
How does GPS work?
The Global Positioning System,
or GPS, comprises 24 satellites along with supporting ground
stations that provide the satellites with continuously updated
information on their precise location. Each satellite transmits
a unique string of binary digits. A GPS receiver is able to determine
the delay in the transmissions, which travel at the speed of
light, and from this calculate its distance from each of several
By triangulating the signals
from four or more satellites, the GPS receiver is able to determine
its position in three-dimensional space. The receiver then converts
this position to longitude, latitude, and altitude. The receiver
also calculates speed from the Doppler shift in the satellites'
transmissions, and direction of travel.
For a more detailed explanation
of GPS, click here.
What is WAAS, and how does it work?
The Wide Area Augmentation System,
or WAAS, enhances the accuracy of GPS by measuring and correcting
for errors caused by variations in the ionosphere. The system
uses a number of precisely surveyed ground stations equipped
with GPS receivers. These stations continuously compare their
GPS locations to their known surveyed locations and determine
the error in the GPS location. The ground stations are connected
to satellite uplinks which transmit an error correction factor
to two geostationary satellites which then beam it back down
to earth. A WAAS-enabled GPS receiver can receive these error
correction factors and apply them to its own GPS positions to
determine its location with even greater accuracy.
For a more detailed explanation
of WAAS, click here.
How accurate is GPS? How much better
Normal GPS is accurate to within
10 meters, or about 33 feet, with 95% confidence. With WAAS this
accuracy is improved to within 3 meters, or about 10 feet, with
What do the LEDs on the transmitter
The LED on the left flashes green
whenever the transmitter is transmitting. The LED on the right
flashes red whenever the 4-way-red command is invoked. The LED
in the center is green when the unit is receiving good GPS data
and red when it is not. If it is dark, that means the transmitter
is not getting power.
What do the switches on the transmitter
The switch on the left is the
Override On/Off. It will turn the transmission on or off regardless
of the state of the lightbar.
The switch on the right is for
extended-range four-way-red preemption. Pushing the switch up
while the transmission is on will tell the intersection receiver
to extend the preemption distance by approximately 50% and turn
the lights to red in all directions when the vehicle reaches
that distance. If the intersection is already in a 1-way-green
preemption, it will change the green light to yellow before going
to 4-way red. Pushing the switch down while the four-way-red
preemption is on will return the transmitter to normal one-way-green
If the transmitter is off, pushing
the 4-way-red switch up will start a 4-way-red preemption.
What is around-the-corner preemption?
enables a Priority
receiver to preempt the traffic signals whenever a vehicle is
close enough, even before it turns onto one of the intersecting
streets. This allows more time for a yellow light cycle before
the vehicle reaches the intersection. This feature works with
both 1-way-green and 4-way-red preemptions.
What happens when two vehicles approach
the same intersection at the same time from different directions?
First, it should be noted that
this situation can occur with or without a preemption system,
and drivers should always be aware of this possibility.
Priority Command provides four levels of high-priority
preemption plus two levels of low-priority preemption. The priority
level is set with a DIP switch inside the vehicle transmitter
case. It is recommended that priority levels be assigned as follows:
Level one: Heavy fire vehicles
such as pumpers, tankers, and ladder trucks
Level two: Medium trucks such
as ambulances and rescue vehicles
Level three: Light trucks, SUVs,
and cars such as police cruisers
Level four: Lower-priority emergency
vehicles such as chiefs' cars and prisoner transports.
Level five: Express busses, snow
plows, and tow trucks responding to police calls.
Level six: Local busses.
When two vehicles of different
priority levels approach the same intersection at the same time,
the higher-priority level will receive the preemption. An ambulance
will have priority over a police car, and a large fire truck
will have priority over an ambulance. Any emergency vehicle will
have priority over a transit bus.
Note that the first three priority
levels are assigned primarily on vehicle weight rather than department
or some subjective judgement of importance. The rationale is
that a large fire truck takes much longer to stop and get back
up to speed than a police car, so it's best to keep it moving
When two vehicles of the same
priority level approach the same intersection, it is impossible
to predict which of the two will get the preemption. Both vehicles
are transmitting at the same time on the same frequency, and
the receiver will always lock on the stronger signal, which will
usually be from the closer vehicle. This is not by design; this
is a law of physics. The vehicle with the stronger signal will
get the preemption.
What happens when two or more vehicles
approach the same intersection in a convoy?
If the vehicles are set to different
priority levels, the intersection receiver will hand off the
responsibility for cancelling the preemption to the trailing
Can the system fail? What happens
if it does?
The Priority Command system uses proven technologies, quality components,
and state-of-the-art manufacturing processes. However, the fact
is that any electronic system can fail for any number of reasons.
Power outages, lightning, radio interference, or component failures
could all cause the system not to function. Additionally, GPS,
although highly reliable, could be unavailable in some locations
or at certain times.
As with any preemption system,
Priority Command cannot guarantee a green light every
single time; however, the system can greatly reduce, if not completely
eliminate, red lights encountered during emergency runs. In the
rare cases in which drivers may encounter red lights, they should
follow the same procedures they used before the system was installed
and proceed with due caution.
How secure is it? Can anyone buy
or make a transmitter?
The Priority Command system is highly secure. Data from the vehicle
transmitter is encrypted using exclusive ChronoKey®
time-based encryption. Even if someone were to record a transmission
and replay it, it would not be accepted by the receiver to preempt
the traffic signals because the authentication key changes every
Since transmitters are sold only
to government agencies, each jurisdiction can set its own policy
regarding who may have transmitters and how they may be used.
For instance, some cities may choose to issue low-priority transmitters
to emergency responders or physicians for their personal vehicles
when they are on call. Additionally, tow truck and snowplow operators
can be issued low-priority transmitters to use as authorized.
How much does it cost?
Priority Command is by far the lowest priced preemption
system on the market today. Please contact us for exact prices
including accessories, installation, and necessary services such
as programming your traffic signal controller. On average, after
installation and setup, the Priority
Command system will
be less than half the price of competing systems.
Return to top.