How Traffic Lights Work Without Power

They generally don’t—unless they have a built-in backup. When the grid goes down, most signals either go dark or switch to a fail-safe flashing mode powered by a cabinet battery backup (UPS). Where no backup exists, the intersection must be treated as an all-way stop, and agencies may deploy officers, portable signals, or generators until power returns.

What Actually Happens When Power Is Lost

Modern traffic signals are controlled by a roadside cabinet that houses the controller (the “computer”), load switches, and a safety device called a Malfunction Management Unit (MMU) or conflict monitor. If power is cut and no backup is present, the lights go dark and the controller shuts down. If a battery-backed uninterruptible power supply is installed, the MMU typically forces a low-power, fail-safe flash to prevent conflicting movements and extend runtime.

Default Fail-Safe Behavior

Intersections are engineered to fail safe under abnormal conditions. Depending on equipment and policy, you may encounter several predictable modes when utility power is unavailable.

• Dark signal: No indications illuminated. This happens when there is no backup power or the backup is depleted.
• All-red flash: Every approach flashes red, effectively creating an all-way stop while conserving energy.
• Red/yellow flash split: Main street flashes yellow (proceed with caution), side street flashes red (stop, then go when clear). This is used where policies allow and battery capacity permits.
• Manual control: Police or traffic officers may direct traffic on site, overriding automated operation.

These modes are designed to prevent conflicting right-of-way indications and to keep traffic moving as safely as conditions allow, prioritizing caution over throughput.

How Lights Can Still Operate: Backup Power Options

Cities often equip key intersections with backup systems so signals can keep working during outages. The most common solutions are cabinet-mounted uninterruptible power supplies (UPS), solar-powered signals (permanent or trailer-mounted), and, in some cases, portable generators.

Uninterruptible Power Supplies in Signal Cabinets

A traffic signal UPS sits inside or beside the cabinet and switches to batteries within milliseconds of a grid failure. Because modern LED signal heads draw far less power than old incandescent bulbs, a well-sized UPS can maintain flashing or even limited fully actuated operation for hours. Agencies typically favor flashing operation during outages to stretch runtime and reduce the risk of conflicts.

Several design choices determine how long a UPS can run and what it powers at an intersection.

• Battery chemistry and size: Lead–acid is common and cost-effective; lithium iron phosphate (LiFePO4) offers longer life, better cold-weather performance, and deeper usable capacity. Runtime commonly ranges from about 2–8 hours, and can exceed 24 hours with larger banks and conservative flashing plans.
• Load management: Controllers may power only the MMU and signal faces needed for flashing, disabling heaters, fans, and some detection to conserve energy.
• Health monitoring: Many UPS units report voltage, temperature, and remaining runtime to the traffic management center via cellular or fiber, triggering crew dispatch when capacity is low.
• Standards and safety: Cabinets and MMUs typically follow NEMA TS-2 or ATC standards; installations adhere to electrical codes (e.g., NFPA 70) and use UL-listed UPS components for reliability.

With LEDs and smart load management, UPS-backed flashing is now the default at many priority intersections, providing predictable operation and buying time for repair crews.

Solar-Powered and Portable Signals

Some intersections, work zones, or rural locations use solar panels charging on-site batteries to operate signals off-grid or to extend runtime during outages. Trailer-mounted, solar-assisted signals are commonly deployed for temporary control during construction or after storms, and can run for days if solar exposure is adequate.

Generators and Microgrids

Portable generators can power high-priority intersections during extended blackouts, though they require fuel, noise management, and on-site setup. Permanent automatic transfer switches are rare at single intersections but are appearing in small “microgrid” pilots where clusters of signals, lighting, and communications gear share backup generation or local storage for resilience.

What Drivers Should Do at a Dark or Flashing Intersection

When you reach an intersection affected by an outage, the rules are designed to keep everyone safe while traffic control is degraded. The exact law can vary by jurisdiction, so follow posted rules and official directions if present.

• Dark signal: In most U.S. jurisdictions, treat it as an all-way stop—come to a complete stop, proceed in turn, and yield to pedestrians; check local law.
• Flashing red: Stop completely, then proceed when it’s safe, yielding to any vehicle that arrived first and to pedestrians.
• Flashing yellow: Proceed with extra caution and be prepared to yield.
• Obey officers: Law enforcement or flaggers on site override the signal indications.
• Mind crosswalks and bikes: Expect detection systems to be offline; give vulnerable road users extra space and time.
• Don’t block the box: Enter the intersection only if you can clear it, even during flashing operation.
• Yield to emergency vehicles: As always, give right-of-way to responding units with lights and sirens.

These practices reduce crash risk and keep limited traffic flowing until normal control returns.

How Agencies Prepare for and Manage Outages

Transportation departments plan for outages just as utilities plan for storms. Their goal is to maintain safety, prioritize critical corridors, and restore coordination quickly.

Agencies typically use a mix of policy, hardware, and operations to improve resilience.

• Prioritization: Critical intersections (near hospitals, freeway ramps, major arterials) get UPS first and more frequent maintenance.
• Preventive maintenance: Regular battery testing and replacement cycles, especially before severe-weather seasons.
• Telemetry and alerts: Real-time notifications when a cabinet goes to flash or battery falls below thresholds, enabling rapid dispatch.
• Preprogrammed flash plans: Standardized flashing patterns that minimize conflicts and conserve energy.
• Portable assets: Stockpiles of trailer signals, generators, temporary stop signs, and cones for rapid deployment.
• Public communication: Push alerts and signage to inform drivers about widespread outages and detours.

With these measures, agencies can keep key locations operating safely and shorten disruption during grid failures.

What Happens When Power Returns

Restoring a signal isn’t as simple as flipping a switch. Controllers must boot, check safety, and rejoin corridor timing plans. Some systems intentionally remain in flash for a short time to stabilize traffic before resuming normal phasing.

The transition back to normal service typically follows a set of automated steps.

• Self-test and conflict check: The MMU verifies there are no faults or conflicting indications.
• Detection re-initialization: Vehicle and pedestrian detectors recalibrate; some may be temporarily offline.
• Timing recovery: The controller resynchronizes with adjacent signals or a central system to restore coordinated “green waves.”
• Technician override if needed: If a fault persists, the cabinet may stay in flash until a technician clears it.

This controlled process helps prevent sudden conflicts and smooths the return to normal traffic patterns.

Key Takeaways

Without power, most traffic lights either go dark or switch to a battery-backed flashing mode designed to be fail-safe. UPS-equipped cabinets, solar or trailer signals, and—when needed—generators keep critical locations operating. Drivers should treat dark signals as all-way stops in most jurisdictions and obey flashing indications or officers on scene. Agencies plan, prioritize, and monitor to maintain safety and restore normal operation quickly after outages.

How are temporary traffic lights powered?

Options: battery, solar, and mains power
Temporary traffic signals rely on various power sources to ensure uninterrupted operation. Battery-operated portable traffic lights offer reliability and ease of use, making them suitable for short-term projects.

What to do if a traffic light has no power?

Fortunately, the law gives drivers directions on how to handle a non-functioning traffic signal. You’re correct in your belief; you treat it as an all-way stop. After stopping, yield to drivers already in the intersection, and if two drivers arrive at the same time, the one on the left yields to the one on the right.

How do traffic lights work during a power outage?

During a short outage the signal will revert to normal utility. Power once service is restored with no disruption of normal signal operations. During an extended power outage once the battery reaches

What batteries do temporary traffic lights use?

What traffic signal is right for me?

Features	Rapid Response	CityLight
Voltage	12v	12v
Battery Box Size	N/A	Standard
No. of Batteries	1	3
Battery Type	Lithium	Lead Acid / AGM