How Traffic Lights Detect Emergency Vehicles

Traffic lights detect emergency vehicles using dedicated preemption systems that listen for sirens, watch for encoded optical/infrared strobes, or receive authenticated GPS/radio messages from the vehicle; once validated, the signal controller accelerates or extends a green for the emergency approach while safely clearing conflicting traffic. In practice, agencies deploy a mix of technologies—optical/IR line-of-sight, acoustic detection, and radio/GPS- or V2X-based messaging—to cut response times and reduce intersection conflicts, all under strict safety and authentication rules.

The core idea: signal preemption vs. priority

Signal preemption interrupts the normal traffic signal plan to immediately serve an emergency vehicle, while signal priority makes smaller adjustments (such as extending a green) without fully breaking coordination. Emergency Vehicle Preemption (EVP) typically provides the highest level of control—triggering red clearance for conflicting movements, withholding new pedestrian phases, and granting an early or extended green—then smoothly returns the intersection to its normal timing once the vehicle clears.

Detection technologies used today

Cities use several complementary methods to identify approaching emergency vehicles. Each has trade-offs in range, reliability, cost, and susceptibility to weather or line-of-sight limitations. Below are the most common approaches you’ll find in modern deployments.

• Optical/IR strobe detection (line-of-sight): A roof-mounted emitter on the emergency vehicle flashes an encoded pattern—often infrared LED pulses—detected by sensors on the signal mast. Systems like Opticom-style IR have ranges typically up to about 1,500–2,000 feet in urban conditions and work best with clear line-of-sight. Modern versions use encrypted, vehicle-specific codes to prevent spoofing.
• Acoustic siren detection: Microphone arrays near intersections analyze siren signatures (wail/yelp) to infer the presence and direction of an emergency vehicle. Newer implementations use digital signal processing and machine learning to reduce false positives, but performance can be degraded by wind, heavy traffic noise, or insulated cabins. Often used as a supplemental layer rather than the sole trigger.
• Radio/GPS (cellular or dedicated RF): A transponder on the vehicle shares authenticated GPS position and heading via cellular or private RF to a traffic management platform, which requests preemption at upcoming signals based on geofenced rules and estimated time of arrival. Because this method isn’t line-of-sight, it can stage a “green wave” well in advance (often 0.5–3 miles), coordinating multiple intersections. Many systems use secure, whitelisted IDs with end-to-end encryption.
• V2X (Vehicle-to-Everything: DSRC/C‑V2X): Using standards-based messages (e.g., SAE J2735 Signal Request Message), vehicles directly request priority from equipped intersections. North American pilots are increasingly C‑V2X-based, integrating with SPaT/MAP broadcasts for status and geometry. This reduces infrastructure add-ons and supports cross-agency interoperability.
• Short-range beacons/RFID: Some corridors use localized beacons that recognize authorized vehicles at close range—useful where cellular coverage is poor or as a backup to other methods.
• Video analytics: AI-enabled cameras can detect flashing light bars or recognize siren patterns, serving as a supplemental trigger. Performance varies with weather, occlusion, and nighttime glare, so agencies tend to pair it with authenticated methods.
• What isn’t used to identify “emergency” status: Inductive loops and basic radar detect presence and speed but cannot, by themselves, verify that a vehicle is an authorized responder.

In many cities, two or more technologies run in parallel—optical/IR for immediate line-of-sight activation, plus GPS/radio or V2X for earlier, networked coordination—improving reliability across diverse environments.

How the preemption sequence works

While timing plans and safety policies vary by jurisdiction, most preemption events follow a repeatable sequence to validate requests, clear conflicts, and then restore normal operation with minimal disruption.

1. Detection: The controller or traffic management system receives a request via optical/IR, acoustic, radio/GPS, or V2X.
2. Validation and authentication: The request is checked against a whitelist or certificate to ensure it’s from an authorized vehicle and approach, often with priority levels (e.g., fire/EMS over transit).
3. Phase selection: The controller determines the target movement (e.g., northbound through/left) and whether to grant an “early green” or extend an active green.
4. Conflict clearance: Conflicting approaches are given yellow then an all-red interval; pedestrian phases not yet started are withheld. Active pedestrian intervals typically run to minimum safety times rather than being cut off mid-crossing.
5. Serve the emergency movement: A green is displayed for the emergency approach, often held until the vehicle is detected as having cleared the intersection or a maximum preemption time is reached.
6. Multiple-vehicle arbitration: If several authorized vehicles request service, the controller prioritizes by class, proximity, or arrival time, resolving ties per policy (NTCIP 1211 supports these rules).
7. Exit and recovery: The controller transitions back to coordination, restoring offsets and cycle lengths smoothly to limit residual delay.

This logic balances urgency and safety: it minimizes conflict risk while preventing gridlock by returning the corridor to its planned timing promptly after the event.

Security and authentication

Modern EVP systems use encrypted codes, digital certificates, and whitelisting to block spoofing. Early-generation optical systems were vulnerable to unauthorized “strobe” devices; today’s implementations employ rolling codes and authenticated identifiers, plus audit logs tying each preemption to a specific unit and time. Networked (cellular/V2X) systems further secure messaging end-to-end and can centrally revoke lost or decommissioned devices.

Benefits and trade-offs

Preemption can dramatically reduce response times and intersection crash risk, but it also temporarily disrupts coordination and can increase delay for other users. The points below summarize the typical advantages and considerations agencies weigh.

• Benefits: Faster emergency response, fewer conflict points at intersections, potential “green wave” staging across multiple signals, and better crew safety.
• Trade-offs: Temporary delays for cross traffic and pedestrians, recovery time to restore coordination, and the need for ongoing maintenance and cybersecurity.
• Reliability factors: Line-of-sight limitations for optical systems, weather and noise impacts on acoustic methods, and the need for resilient backhaul for cloud or cellular-based solutions.

When designed with redundancy and clear policies, EVP delivers significant safety and performance gains while keeping wider network impacts manageable.

How cities choose a system

Selection depends on corridor geometry, climate, fleet mix, and existing infrastructure. Agencies aim for interoperability across jurisdictions so mutual-aid vehicles are recognized everywhere, and they balance deployment speed with lifecycle costs and cybersecurity requirements.

• Cost ranges (typical): Intersection equipment: roughly $3,000–$10,000+ per location (sensors, wiring, controller firmware); vehicle units: about $1,500–$5,000 each; networked/cloud platforms may add annual fees.
• Environment: Dense urban canyons favor GPS/radio or V2X to overcome line-of-sight issues; suburban arterials often do well with optical/IR supplemented by radio.
• Maintenance: Optical sensors require alignment and periodic cleaning; radio/GPS systems need data connectivity and firmware updates; all systems benefit from regular audits and event log reviews.
• Standards and compatibility: Support for NTCIP 1211 (signal control priority/preemption), and, for V2X, SAE J2735 (SRM/SSM, SPaT/MAP) to future-proof investments.

Many agencies start with optical/IR for quick wins on critical corridors, then layer GPS/radio or V2X for network-wide coordination and analytics.

Real-world usage and range

Large U.S. metros commonly deploy optical/IR systems at thousands of intersections, with typical activation distances around 1,500–2,000 feet depending on visibility. Increasingly, statewide and regional programs add GPS/cellular or C‑V2X-based EVP to coordinate across city boundaries and to pre-stage greens several intersections ahead. Pilots since 2023–2025 in states like Utah, Arizona, Florida, Michigan, and others are demonstrating cloud- and V2X-enabled EVP that integrates with existing controllers and central systems.

Future trends

EVP is moving toward connected, standards-based operations that work across vendors and jurisdictions while strengthening security and analytics.

• C‑V2X integration: Direct, authenticated Signal Request Messages from vehicles to signals, with Signal Status feedback, improving reliability and reducing hardware at the mast arm.
• Cloud coordination: Corridor-level optimization that sequences multiple intersections, leveraging GPS telemetry and predicted arrival times.
• AI-enhanced sensing: Better acoustic and video recognition to supplement authenticated requests in complex environments.
• Cybersecurity hardening: Wider use of PKI, device attestation, and centralized revocation to mitigate spoofing or tampering.
• Multimodal policies: Harmonizing emergency preemption with transit/freight priority and railroad preemption without compromising safety.

Together, these trends aim to make preemption faster, more interoperable, and more secure—while minimizing disruption to everyone else on the road.

Summary

Emergency vehicles are detected by traffic signals through a mix of optical/IR strobes, acoustic siren recognition, and authenticated GPS/radio or V2X messages. Once validated, controllers safely clear conflicts and grant a green to the emergency approach, then restore normal timing. Modern deployments emphasize encryption, standards (NTCIP 1211; SAE J2735), and multi-technology redundancy to boost reliability. As cities adopt C‑V2X and cloud coordination, EVP is becoming more predictive, more interoperable, and better protected against spoofing—cutting response times while keeping intersections safe.

How does my car detect emergency vehicles?

Your car might detect an approaching emergency vehicle through systems like Stellantis’ Emergency Vehicle Alert System (EVAS), which uses a cloud-based technology called Safety Cloud by HAAS Alert to send digital alerts to compatible vehicle infotainment systems. Alternatively, some navigation apps, like Waze, use GPS signals from emergency vehicles and user reports to display alerts on a driver’s mobile device. 
How it Works (Stellantis EVAS)

1. Emergency Vehicle Activation: An active emergency vehicle triggers a digital alert through its connected system. 
2. Cloud Transmission: The alert is sent to Safety Cloud, a cloud-based service that acts as a communication hub. 
3. Digital Notification: Safety Cloud then transmits this information to nearby, compatible vehicles. 
4. In-Vehicle Alert: The driver’s car receives the digital signal and provides an alert, often with both an audible beep and an on-screen warning, giving the driver time to safely pull over. 

How it Works (Navigation Apps)

1. Emergency Vehicle GPS: Opens in new tabMany emergency vehicles (such as police, fire, and ambulances) are equipped with GPS and private communication systems that can share their location. 
2. App Integration: Opens in new tabNavigation applications like Waze or other apps can receive this location data. 
3. User-Generated Alerts: Opens in new tabThese apps can also incorporate user reports of emergency vehicles or hazards. 
4. On-Screen or Audible Alerts: Opens in new tabThe app then provides an alert on the driver’s mobile device, warning them that an emergency vehicle is approaching or that there is a hazard ahead. 

Key Factors for Success

• Vehicle Technology: Opens in new tabBoth the emergency vehicle and your car must be equipped with compatible technology for digital alerts to function. 
• Connected Systems: Opens in new tabCloud-based systems like Safety Cloud by HAAS Alert connect vehicles and infrastructure, allowing for communication between them. 
• Active Systems: Opens in new tabThe emergency vehicle’s emergency lights and sirens, coupled with its connected alert system, trigger the warning in your car. 

What is the Massachusetts law on emergency vehicles?

Massachusetts law gives fire trucks responding to an alarm, police patrols and ambulances transporting the sick the right-of-way through “any street, way, lane or alley.” If a driver willfully blocks the way of one of these emergency vehicles, the driver could receive a fine of fifty dollars and a possible jail …

Can you run a red light if an ambulance is behind you?

🚦 If an emergency vehicle is behind you at a red light, don’t cross the white line – it’s a traffic offence! 😮 If you’re at the lights and can see an 🚑 behind you, stay where you are until the light turns green or the emergency vehicle has passed.

How do traffic lights sense emergency vehicles?

When the strobe is activated, it is detected by a sensor at the signal that turns the signal green for the approaching emergency vehicle. Once the emergency vehicle has passed, the signal will return to the pattern it was running before. 5. Can I make a light change to green by flashing my headlights?