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 siren and turn signal switches. 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 to go.

The Priority Command vehicle transmitter can be installed in, on, or under the dash, or in the center console. Or 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 preemptions.

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 signal controllers?

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 flow?

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 traffic flow.

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 factors—intersection size, yellow light time, roadway speed limit, and vehicle speed—to 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 Command distributor can quote an exact price and delivery schedule for custom intersection programming.

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 satellites.

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 is WAAS?

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 95% confidence.

What do the LEDs on the transmitter indicate?

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 do?

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 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 preemption mode.

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?

Around-the-corner preemption enables a Priority Command intersection 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 whenever possible.

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 vehicle.

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 second.

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. Your distributor can quote 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.


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