Traffic signal controllers and conflict monitors: how signals stay safe when parts fail
What a signal cabinet holds, what flashing signals mean and how to report a failure
Safety Behind the Wheel Foundation built this guide from Federal Highway Administration (FHWA) handbooks, guidance and the federal traffic control manual, Insurance Institute for Highway Safety (IIHS) data, the California Department of Motor Vehicles driver handbook and federal regulations. Every source is listed at the end.
What a traffic signal controller does
At a signalized intersection, the controller is the “brain” of the system. FHWA’s guide to signalized intersections uses that word [1]. The controller follows a stored program. It sets when each light changes and how long each movement lasts [1].
Detectors, such as pavement sensors or cameras on a pole, tell the controller that vehicles are waiting [1].
Why does this matter for safety? FHWA’s handbook explains right of way assignment as letting some drivers and walkers go ahead of others by showing signal lights [2]. The national traffic control manual says well justified signals can lower how often and how badly certain crashes happen, especially right angle collisions [3]. A right angle collision is one where a vehicle strikes the side of another vehicle.
The controller also handles special jobs. One is preemption, which means interrupting the normal cycle for a high priority event, such as a train approaching a nearby crossing [1]. Railroad gates and crossing signals are a separate topic, covered in our railroad crossing article.
What sits inside the signal cabinet
The gray metal box near the corner is the signal cabinet. FHWA’s signal program handbook shows its main parts: the controller, a malfunction monitoring unit, flasher units and flash transfer relays [2]. A relay is an electric switch. Cabinets and controllers follow equipment standards called NEMA TS 1 and TS 2, which an industry group publishes [2].
Comparison compiled by Safety Behind the Wheel Foundation
What sits inside a signal cabinet
| Part | Its job in plain words | What can go wrong |
|---|---|---|
| Controller | Runs the timing program and picks which movement goes next [1] | Outdated timing or changes made without a record [3] |
| Detectors | Tell the controller where vehicles are waiting [1] | A sensor fails and the signal serves the wrong movement |
| Conflict or malfunction monitor | Watches the power sent to the lights and forces flash if the mix is unsafe [2] | Wrong settings or no regular test |
| Flasher and flash transfer relay | Make the flashing pattern and switch the signals into it [2] | Worn parts that look fine from the outside |
| Cabinet and lock | Shield the electronics from weather and tampering | An unlocked door or a damaged seal |
The cabinet works as one system. Its parts must match each other.
How a conflict monitor works
Think of the conflict monitor as a second set of eyes on the controller. A bad part, a power surge, a software fault or a wrong setting can cause a controller mistake. The monitor exists so that one mistake does not put two crossing streams of traffic on green at once.
A conflict is a pair of movements that cross paths. North south traffic and east west traffic are the simple example. They must never both see green. The monitor watches the electrical signals going out to the lights. It does this apart from the controller’s program.
If the monitor sees a forbidden combination, it stops normal operation. It then uses the flasher and the flash transfer relays to switch the intersection to flash [2]. Flash is the fallback because drivers already know how to treat flashing red and flashing yellow.
The handbook uses two names: conflict monitor unit (CMU) and malfunction monitor unit (MMU) [2]. Agencies also say “malfunction management unit.” They do the same basic job, but some models check more kinds of faults than others. Depending on the model and setup, a monitor may also catch a signal face that has lost its red light, or a yellow that runs too short. Ask your agency which type it uses.
After a trip into flash, most agencies send a technician to find the cause before normal operation returns. Settings matter as much as hardware. Technicians program the monitor with the pairs of movements that must never be green together. If those settings are wrong, the monitor may miss a true conflict, or trip when nothing is wrong.
What flashing and dark signals mean for drivers
The national traffic control manual sets the flash rules. A driver facing a flashing red light must stop at the stop line and then go only as the rules for a stop sign allow [3]. A driver facing a flashing yellow light may enter the intersection, but only with care [3].
After a monitor trip, you may see flashing red on some approaches and flashing yellow on others, or flashing red in every direction. Obey the color facing you. Do not assume the other driver sees the same one.
A fully dark signal is different. Rules come from state law and driver manuals. California’s handbook says to stop as if every direction has a stop sign, then proceed carefully when it is safe [4]. Safety Behind the Wheel Foundation gives drivers the same advice. Other states publish their own wording, so check your state’s manual.
In every case, slow down early and look for cross traffic. Report the outage to the road agency or the local nonemergency line. Call 911 if a collision or immediate danger is present.
Why this equipment matters for preventing collisions
FHWA says roughly one quarter of traffic deaths and about half of traffic injuries in the United States are tied to intersections [5]. Signalized intersections account for about one third of intersection deaths [5]. FHWA’s table, built from federal crash data, counts 4,204 deaths in 2022 in collisions involving a signalized intersection, up from 3,347 in 2018 [5].
Red light running is a large part of that picture. In 2024, 1,119 people died in collisions that involved it [6]. Half of those killed were pedestrians, bicyclists and people in other vehicles who were hit by the red light runner [6].
These numbers describe crash patterns. They do not show that equipment failed in each case. The equipment’s job is to give clear, correct and steady instructions, and to fail into a safe state. Several timing choices live in the controller, and some have strong evidence behind them.
Comparison compiled by Safety Behind the Wheel Foundation
Signal timing features and FHWA evidence
| Signal feature | What it does | FHWA evidence |
|---|---|---|
| Yellow change interval | Sets how long yellow shows after green | 36% to 50% less red light running, and 8% to 14% fewer total crashes [7] |
| Backplates with reflective borders | Frame the lit signal so it is easier to see | 15% fewer total crashes, based on a 2005 study [8] |
| Leading pedestrian interval | Lets walkers start 3 to 7 seconds before cars get green | 13% fewer pedestrian vehicle crashes [9] |
A controller cannot fix a poor design. The same manual warns that delay and some crash types can be higher under signal control than under stop sign control [3]. Signals that are not justified can raise disobedience and rear end collisions [3]. That is why the manual requires an engineering study before a signal goes in [3].
Who is responsible for signal equipment
On public roads, an agency chooses, designs, installs and maintains traffic signals. That may be a state transportation department, a county or a city. The national manual says the agency should settle, before installing any signal, who maintains the signal, its hardware, its software and its timing plans [3].
Federal rules set the base standard. The national manual is the standard for all traffic control devices on any street, highway or bicycle trail open to public travel [10]. The 11th edition took effect on January 18, 2024. States must adopt it as their own standard within two years [11].
Several manual rules affect controllers directly. An engineering study must come before a new signal [3]. The agency should keep each controller on its planned timing schedule and check it often. It should record every timing change and let only authorized people make them. It should also plan for alternate operation during a failure, such as flashing mode or manual traffic direction [3].
Residents, parents and employers should never install, open or adjust signal equipment. Ask the road agency instead. Cabinet security matters too: the handbook notes that one state transportation department is adding electronic locks to its cabinets [2].
Campuses and private road owners face the same questions. If a road is open to public travel, the national standard may apply [10]. A licensed traffic engineer can confirm what applies before any equipment is bought.
How agencies buy and specify signal equipment
Few agencies build this equipment themselves. The usual channels are:
- A traffic engineering firm studies the site and designs the signal and its timing plans.
- An infrastructure supplier provides cabinets, controllers, monitors and detectors.
- An approved installation contractor, often chosen from a qualified list, installs and tests the equipment.
The work needs skilled design, programming, maintenance and testing. An FHWA guidebook prompts agencies to ask whether they have standard specifications for equipment such as the controller, cabinet and detectors [12]. FHWA describes weaker signal programs as ones that treat signals like commodities instead of long term resources [2].
Keeping the equipment working: testing and upkeep
Parts age, detectors fail and settings change. FHWA’s 2015 guidebook offers example goals that agencies can adapt. They are examples, not rules. One is to visit every intersection once every two months for preventive maintenance. Another is to test items that damage can hide from the eye, on a regular schedule [12].
The guidebook also suggests carrying enough spare safety critical parts on each maintenance vehicle to restore safe operation within two hours. Another example is stocking spares to restore full operation at up to three intersections within 24 hours, and keeping accurate as built records [12].
Monitor testing has its own track record. An FHWA staffing report describes a 2007 national report card on signal practice. That card listed annual conflict monitor testing, with formal documentation for some or all equipment, as part of regular preventive maintenance [13]. FHWA’s handbook lists a monitor tester, noted as required for annual industry certification, among example equipment [2]. The staffing report also summarizes a guide from 2000 that estimated an average intersection needs about 60 hours of maintenance a year, with 70% of it preventive [13].
Modern controllers also keep logs. An FHWA study of automated signal performance tools lists tracking occurrences of intersection flashing operation, and finding failed detectors, among its measures [14].
One reasonable order for a maintenance visit looks like this:
- Read the controller’s event log and note any recent flash events or faults.
- Inspect the cabinet for moisture, damage, loose wiring and a working lock.
- Test the conflict or malfunction monitor with its tester, and confirm the flash pattern.
- Compare the running timing with the written timing record.
- Check detectors and signal heads at the intersection itself.
- Record what was found, what was repaired and what parts were used.
Limits and common mistakes
A conflict monitor is a safety net. It does not replace good design. Watch for these errors:
- Treating the monitor as a timing fix. It catches faults, not poor timing. A signal can be free of conflicts and still invite red light running. That is why yellow intervals are set with care [7].
- Changing hardware without updating settings. A new controller or monitor needs matching, documented settings.
- Skipping tests because the signal looks fine. Some damage cannot be seen by eye [12].
- Poor records. Without a log of timing changes, nobody can tell what changed [3].
- Ignoring repeat flash events. Repeated trips are a warning, and event logs make them visible [14].
- Using complaints as the only measure. FHWA names this habit among the traits of weaker signal programs [2].
- Having no outage plan. The manual expects agencies to provide alternate operation during a failure [3].
What you can do as a driver, neighbor or organization
As a driver, treat any flashing or dark signal as a cue to slow down. Then report it. Give the cross streets, your direction of travel, the time and what the lights were doing.
As a resident, parent, school board member or small employer, you can ask your road agency a few plain questions:
- Who is responsible for the signal near our school, worksite or home?
- How often is the monitor tested, and is the result recorded?
- How does the agency learn about outages, and how fast does it respond?
- Does the agency keep a log of flash events at this intersection?
You can also ask the agency to review a location that worries you. Local crash data helps make the case.
If you or your family were hurt in a car accident at a signal, tell the police what you saw of the lights. Ask the road agency how to request signal maintenance records. A lawyer can advise you on your own situation.
Frequently asked questions
What is a conflict monitor in simple terms?
It is a safety device in the signal cabinet. It watches the lights going out to the intersection. If it sees a forbidden combination, such as green for two crossing streams, it forces the signal into flash [2]. It works apart from the controller’s timing program, so one fault should not defeat both.
Why is the signal flashing red in every direction?
All way flashing red often means the signal went into flash because of a fault, a power problem or planned work. A driver facing flashing red must stop and then follow the rules for a stop sign [3]. Look for cross traffic, take turns, and report the problem if no crew is on site.
Can a conflict monitor fail?
Yes, as any electronic part can. Its settings can also be wrong, which is why agencies test monitors on a schedule and keep records of the results. A test confirms the monitor still trips when it should. One FHWA report cites annual conflict monitor testing as a regular maintenance practice [13].
Who should I call about a dark or flashing signal?
Start with the city, county or state agency that owns the road. If you cannot tell who owns it, use the local nonemergency number. Call 911 for a collision, injury or immediate danger. Do not open or touch the cabinet, even if its door looks loose. Report that too.
Do campuses and private roads need the same equipment?
It depends. The national manual is the standard for any street, highway or bicycle trail open to public travel [10]. A private road that the public can use may fall under it. A licensed traffic engineer can tell you what applies. An engineering study may even point to a better choice than a signal [3].
How often should signal equipment be tested?
The sources reviewed here offer examples, not one national schedule. FHWA’s guidebook suggests visiting each intersection every two months [12]. A national report card named annual conflict monitor testing as part of preventive maintenance [13]. Ask your agency for its own written schedule.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation urges drivers to slow down at every dark or flashing signal and report it, and urges road agencies to test and record their signal equipment on a schedule. A signal controller and its conflict monitor do quiet work. The controller sets the order and length of each movement. The monitor stands guard so that a fault turns into a flash, not a crash. Both depend on trained people who design, program, test, record and repair.
You do not need an engineering degree to help. Slow down at flashing and dark signals. Report them. Ask your road agency how it tests its equipment and how fast it responds. Public signals belong in the hands of authorized agencies, and good questions from residents help them do the job well.
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- Speed camera and red light camera laws by state: 2026 guide
- Railroad crossing safety: signs, gates and what to do if you're stuck
- Emergency vehicle signal preemption: how responders get a safe green light
Sources and further reading
- 1U.S. Department of Transportation, Federal Highway Administration. Signalized Intersections Informational Guide, Second Edition (FHWA-SA-13-027). Published July 2013. highways.dot.gov/…/fhwasa13027.pdf
- 2U.S. Department of Transportation, Federal Highway Administration. Traffic Signal Program Handbook (FHWA-HOP-23-041). Published April 2023. ops.fhwa.dot.gov/…/fhwahop23041.pdf
- 3U.S. Department of Transportation, Federal Highway Administration. Manual on Uniform Traffic Control Devices, 11th Edition, Part 4: Highway Traffic Signals. Published December 2023. mutcd.fhwa.dot.gov/pdfs/11th_Edition/part4.pdf
- 4California Department of Motor Vehicles. California Driver Handbook, Section 7: Laws and Rules of the Road. Updated June 11, 2025. dmv.ca.gov/…/laws-and-rules-of-the-road
- 5U.S. Department of Transportation, Federal Highway Administration. About Intersection Safety. Updated July 28, 2026. Data year: 2018 to 2022 (NHTSA FARS data). highways.dot.gov/…/about
- 6Insurance Institute for Highway Safety. Red light running. Updated July 2026. Data year: 2024. iihs.org/research-areas/red-light-running
- 7U.S. Department of Transportation, Federal Highway Administration. Yellow Change Intervals (Proven Safety Countermeasures). Undated web page, reviewed October 2026. highways.dot.gov/…/yellow-change-intervals
- 8U.S. Department of Transportation, Federal Highway Administration. Backplates with Retroreflective Borders (Proven Safety Countermeasures). Undated web page, reviewed October 2026. Data year: 2005 (study cited by FHWA). highways.dot.gov/…/backplates-retroreflective-borders
- 9U.S. Department of Transportation, Federal Highway Administration. Leading Pedestrian Interval (Proven Safety Countermeasures). Undated web page, reviewed October 2026. highways.dot.gov/…/leading-pedestrian-interval
- 10Electronic Code of Federal Regulations. 23 CFR Part 655, Subpart F, Section 655.603 Standards (Traffic Control Devices on Federal-Aid and Other Streets and Highways). Current text, reviewed October 2026. ecfr.gov/…/subpart-F
- 11U.S. Department of Transportation, Federal Highway Administration. Manual on Uniform Traffic Control Devices (MUTCD). Updated August 5, 2026. mutcd.fhwa.dot.gov
- 12U.S. Department of Transportation, Federal Highway Administration. Traffic Signal Management Plans, Chapter 4: Outline for Traffic Signal Management Plans (FHWA-HOP-15-038). Published November 2015. ops.fhwa.dot.gov/…/ch4.htm
- 13U.S. Department of Transportation, Federal Highway Administration. Traffic Signal Operations and Maintenance Staffing Guidelines (FHWA-HOP-09-006). Published March 2009. Data year: 2000 and 2007 (guide and report card it summarizes). ops.fhwa.dot.gov/…/fhwahop09006.pdf
- 14U.S. Department of Transportation, Federal Highway Administration. A Methodology and Case Study: Evaluating the Benefits and Costs of Implementing Automated Traffic Signal Performance (FHWA-HOP-20-003). Published April 2020. ops.fhwa.dot.gov/…/fhwahop20003.pdf
This article reflects the safety priorities of safe intersections and well maintained traffic signals. It is general education, not legal, insurance or engineering advice, and it is not a design guide. Rules change, so verify details with official sources. For a signal problem, contact the road agency that owns the signal. For a legal or insurance question after a car accident, contact a licensed attorney or your insurer. For a signal on private property, consult a licensed traffic engineering firm. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Traffic signal controllers and conflict monitors: how signals stay safe when parts fail.” drivewithcare.org, https://drivewithcare.org/traffic-signal-controllers-conflict-monitors/. Reviewed October 2026.
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