Intelligent Roadway Information System

Gate Arms

Select View ➔ Gate Arm Arrays menu item

Gate arms are traffic control devices which restrict access to a section of roadway. They can be used, for example, for reversible lanes which can be changed by time of day.

Gate arms are grouped as arrays of 1-8 arms. Opening or closing an array will control all gate arms in the array.

Verification Cameras

One or two cameras can be associated with a gate arm array. This allows operators to check traffic conditions and verify the status before opening or closing the gates. The second camera can be used to monitor approaching traffic. The Swap button allows the camera images to be swapped between larger and smaller views.

Warning Action Plan

An action plan can be associated with a gate arm array, allowing one or more DMS to display appropriate warning messages. Whenever the array state changes, this plan will immediately be changed to the appropriate phaseWarning Open Phase when fully open, otherwise Warning Closed Phase.


Great care must be taken to prevent traffic conflicts when operating gate arms. Two types of interlock are available for this purpose. An open interlock is a constraint which prevents the gate arm from being opened. Similarly, a close interlock prevents the gate arm from being closed.

When a gate arm is open, all other gate arms on the same roadway, but in any other direction will have an open interlock.


Each gate arm array can be assigned a prerequisite array (on the same roadway and direction). This configuration prevents the gate arms from opening until the prerequisite has been opened, using an open interlock. Once an array and its prerequisite are both open, the prerequisite array will have a close interlock until the other array is closed.


An open conflict exists when an open interlock constraint is broken. Similarly, a close conflict exists for close interlock constraints. IRIS will not automatically try to resolve conflicts.

If a conflict is detected, an alert email will be sent to the address in the email_recipient_gate_arm system attribute.

If communication is lost to a gate arm, the state will be unknown. After an interval equal to the gate_arm_alert_timeout_secs system attribute, it will be treated as possibly open, and interlock conflicts will be checked.


There are a couple of extra security features to restrict access to gate arm control.


There is a whitelist of client IP addresses from which clients are allowed to control gate arms. It is specified as the gate.arm.whitelist property in the /etc/iris/ configuration file. The property contains a list of addresses in CIDR notation (exact IP, or ranges specified such as

System Disable

Another gate arm security feature causes the entire gate arm system to be disabled whenever any configuration change is made to a gate arm. This includes any changes to a gate arm array, controller or associated comm link. IRIS will not send any command to open or close any gate arm while in this state. The only way to re-enable gate arm control is to create a file in the server filesystem at /var/lib/iris/gate_arm_enable (using the touch command).