ISO 26684: Cooperative Intersection Signal Info and Violation Warning (CIWS)

1. Overview of ISO 26684

ISO 26684:2015 defines the concept of operations, system requirements, and test procedures for Cooperative Intersection Signal Information and Violation Warning Systems (CIWS). Developed by ISO/TC 204 for Intelligent Transport Systems, this international standard addresses one of the most dangerous scenarios in road traffic: signalized intersection violations that lead to severe injuries, fatalities, and property damage worldwide. According to traffic safety studies, intersection-related crashes account for a significant percentage of urban road fatalities, making systems like CIWS a critical component of the Vision Zero approach to road safety.

The CIWS concept leverages Vehicle-to-Infrastructure (V2I) communication to provide drivers with real-time traffic signal phase information and, when necessary, imminent violation warnings. The system architecture consists of Roadside Equipment (RSE) installed at signalized intersections that broadcasts signal status data including phase, timing, and location information, and Onboard Equipment (OBE) within vehicles that processes this data combined with vehicle telemetry to determine whether a warning should be issued to the driver.

2. System Architecture and Functional Requirements

2.1 CIWS Classification

2.2 State Machine Design

The CIWS operates through a well-defined state machine with four primary states: RSE Inactive, RSE Active, OBE Inactive, and OBE Active. Within the active state, the system further distinguishes between Warning and No Warning sub-states. Six precisely specified transition criteria govern state changes:

• Criterion 1 (RSE Activation): RSE becomes active when operator or automatic system turns it on at the intersection.
• Criterion 2 (RSE Deactivation): Triggered by operator action, system failure detection, or maintenance mode engagement.
• Criterion 3 (OBE Activation): Activated when the vehicle ignition or accessory switch is turned on.
• Criterion 4 (OBE Deactivation): Deactivated when ignition is turned off or during detected system failures.
• Criterion 5 (Warning Activation – Class II): Triggered when the vehicle enters the warning zone based on TTAI calculations versus remaining green time.
• Criterion 6 (Warning Deactivation): Warning terminates after crossing the stop line or driver action eliminates the violation risk.

3. Warning Threshold Algorithms

The core technical innovation of ISO 26684 is the mathematically precise warning threshold calculation. The Time to Arrival at Intersection (TTAI) represents how long a vehicle needs to reach the stop line at current speed.

TTAI = X_v / v

The warning logic compares TTAI against the remaining green time (G_r) and yellow signal duration (Y) to determine the appropriate action:

The RSE must be positioned at a location X_AL providing sufficient distance for the fastest expected vehicle to stop safely. The minimum distance calculation incorporates driver perception-reaction time, OBE processing delay, and vehicle braking performance.

4. Test Requirements and Validation

The standard mandates comprehensive testing under controlled conditions to verify CIWS performance before deployment.

Test Environment: A flat, dry asphalt or concrete surface with ambient temperature between 10 and 30 degrees Celsius. The test intersection may be located on a closed test track rather than a public road to eliminate uncontrolled variables. The approach zone must be clearly marked with known distances from the RSE communication point to the stop line.

Test Vehicle: Must be equipped with a precision data recorder capturing CIWS outputs as synchronized functions of time and vehicle position. GNSS data logging at minimum 10 Hz update rate is recommended for accurate trajectory reconstruction.

Test Procedure for Class I: One pass/fail run at the design speed verifying that signal phase information is correctly displayed to the driver.

Test Procedure for Class II: Two runs are required – one where the approach timing triggers warning activation and one where it does not. The system must correctly discriminate between these two conditions. The example in the standard uses a 65 km/h approach speed with a 30-second green interval, yielding a warning threshold of 23 seconds after green initiation.

Communication Delay Budget: The standard specifies maximum tolerable delays of 200 milliseconds for OBE communication plus processing and 1.0 second for RSE data broadcast. These strict timing constraints are critical design parameters for real-time embedded system engineers.

5. Engineering Design Insights

ISO 26684 offers several valuable lessons for practicing engineers developing V2X safety systems:

GNSS Independence: While GNSS is the primary position source, the system may also derive location from RSE spot communication zones. This dual-redundancy approach improves robustness in challenging environments such as urban canyons or tunnel approaches where satellite visibility is limited.

Actuated Signal Compatibility: The standard explicitly warns that CIWS should not be installed at intersections with actuated (variable-timing) signals when using spot communication, because unpredictable phase changes due to traffic detection defeat the fixed-threshold warning algorithm.

HMI Design Principles: Warning displays must be intuitive, non-distracting, and properly prioritized among competing in-vehicle alerts. The standard defers detailed HMI design to manufacturers but mandates appropriate arbitration between multiple warnings.

Failure Mode Management: The OBE must autonomously detect system failures and inform the driver that CIWS information is unavailable. This graceful degradation prevents over-reliance on the warning system and ensures drivers maintain appropriate attention levels.

6. Frequently Asked Questions