The Pulse of Transit: EN 15531 (SIRI) Real-Time Standard

Introduction to EN 15531 (SIRI)

When you look at a smartphone app and see “Bus 42: Delayed – Arriving in 3 minutes,” that information didn’t appear by magic. It was likely transmitted from the bus operator’s control center to the app server using a standardized language. EN 15531, known globally as SIRI (Service Interface for Real-time Information), is the European standard that defines this language.

While static schedules tell you what should happen (the domain of NeTEx), SIRI tells you what is actually happening. It allows different computer systems—from vehicle tracking (AVL) to station displays (PID)—to exchange live updates about vehicle locations, delays, and cancellations.

Snippet Definition: What is EN 15531?

EN 15531, or SIRI, is a CEN technical standard specifying an XML-based protocol for the exchange of real-time information for public transport services. It defines the message structures for exchanging data regarding vehicle activity (Vehicle Monitoring), stop arrival predictions (Estimated Timetable), and operational alerts (General Messages) between different computer systems.

Core Functional Services

SIRI is modular. An implementation doesn’t have to support everything; it can pick the specific “services” needed. The most common modules are:

1. Vehicle Monitoring (SIRI-VM)

“Where is the bus now?”

This service transmits the current location (GPS coordinates), speed, and heading of the vehicle. It is essential for map-based visualizations.

2. Estimated Timetable (SIRI-ET)

“When will it get here?”

This service provides the predicted arrival and departure times at specific stops. It updates the “Planned” time with the “Expected” time based on current traffic conditions.

3. Stop Timetable (SIRI-ST)

“What is shown on the station screen?”

Optimized for providing data to station departure boards, listing all vehicles approaching a specific stop point.

4. General Message (SIRI-GM)

“Why is the line closed?”

Transmits unstructured text or structured codes regarding operational changes, such as “Station closed due to construction” or “Snow delay.”

Communication Patterns

EN 15531 supports two distinct ways of moving data:

• Request/Response: The client asks once (“Where is Bus 5?”), and the server answers once. Good for one-off queries.
• Publish/Subscribe (Pub/Sub): The client subscribes once (“Tell me about Line 10”). The server then pushes updates automatically every time something changes (e.g., every 30 seconds). This is the standard for continuous operations.

Comparison: SIRI vs. GTFS-Realtime

In the global transit tech world, two standards dominate.

Relationship with NeTEx

Under the European ITS Directive, SIRI and NeTEx are the “Dynamic Duo.”

• NeTEx (EN 16614): Provides the Planned data (Stops, Routes, Scheduled Times, Fares). It is the static map.
• SIRI (EN 15531): Provides the Real-time deviations from that plan. It references the unique IDs (StopPointID, LineID) defined in NeTEx.