Road weather sensors and 511: how ice, fog and rain reports reach drivers
What road sensors measure, how crews use the data and where to check before a trip
Safety Behind the Wheel Foundation built this guide from Federal Highway Administration (FHWA) road weather guidance, standards and state case studies, National Weather Service (NWS) alert information and published driving research. Every source is listed at the end.
What a road weather system is and why it matters
A road weather information system has three parts. Sensor stations sit along the road. A communication link carries their data. Central computers collect the data from many stations [1]. Each station is called an environmental sensor station, or ESS. It is a fixed roadside spot with one or more sensors that read the air, the pavement or nearby water levels [2].
The aim is a current, local picture of what weather is doing to the road. Of all weather related vehicle collisions, FHWA says 24 percent happen each year on pavement covered in snow, slush or ice [7]. Another 15 percent happen during snowfall or sleet [7]. More than 1,300 people die and over 116,800 are injured in wrecks on that kind of pavement each year [7].
Fog adds its own toll. FHWA counts more than 38,700 collisions in fog, over 600 deaths and more than 16,300 injuries each year [8]. Both FHWA pages give yearly figures without naming the data years, so treat them as a general scale, not a current count.
A good starting point is to ask what decision the data must support. Should a crew spread deicing material now? Should a sign tell drivers to slow down? This article follows that chain, from sensor to decision to the report on your phone.
For how to drive in bad weather, see our guides to snow and ice and fog. This article covers the equipment behind the reports.
What the sensors measure
FHWA sorts the measurements into three groups. Atmospheric sensors read air temperature, humidity, visibility distance, wind, and the type and rate of rain or snow. Pavement sensors read surface temperature, whether the road is wet, icy or flooded, and what chemicals are on it. Some also read conditions below the surface. Water level sensors track streams, rivers and tides near roads [2].
Two terms are worth knowing:
- Freeze point is the temperature at which the liquid on the road would start to freeze. Deicing chemicals lower it, so crews can see whether a treatment still protects the road.
- Subsurface temperature is the temperature under the road surface. It changes more slowly than the surface, so it shows whether the road is warming or cooling.
The table below turns each reading into plain terms, based on the measurements FHWA lists [2].
Comparison compiled by Safety Behind the Wheel Foundation
What each road weather reading means
| Reading | What it tells a road crew | Why it matters to you |
|---|---|---|
| Air temperature and humidity | Whether frost or ice could form | Cold, damp air raises the chance of slick spots |
| Pavement temperature | How cold the road surface is | The road can be colder than the air |
| Pavement condition | Whether the road is dry, wet, icy or flooded | It shows what your tires will meet |
| Freeze point and chemical level | Whether a treatment is still working | It hints at when crews must return |
| Visibility distance | How far drivers can see | It can trigger fog, smoke or dust warnings |
| Wind speed and direction | How hard gusts are hitting | Gusts can push tall vehicles on open stretches |
| Precipitation type and rate | Rain, snow or sleet, and how heavy | Grip can change fast as it shifts |
Where stations go, and why that choice matters
One FHWA map lists 2,499 stations owned by state transportation agencies. The page does not state a data year [9]. That is a lot of equipment, yet each station sees only its own spot. A shaded curve, a river valley or a bridge deck can behave differently from a station a few miles away.
FHWA’s siting guidelines describe two kinds of sites. Regional sites give a broad, real time view of an area and can feed weather prediction models. Local sites watch a specific road segment, bridge or other feature. The guidelines also cover where to place sensors on the tower and in the pavement [3]. The aim is data that fits the purpose of the site, so the money spent on equipment pays off [3].
Moving sensors fill the gaps between stations. Vehicle mounted systems can read pavement temperature, friction and air temperature, then send the data to a central office through GPS linked tracking [2]. Friction here means how much grip the tires have.
Minnesota shows how far this can go. In an FHWA case study, its transportation department had fitted 700 of its 850 plow trucks with devices that log air temperature, humidity, surface temperature, wiper status and brake status. About 250 plow trucks also carried cameras [10].
Calibration and upkeep: the part people overlook
Calibration means checking a sensor against a known reference and adjusting it so its readings are right. FHWA’s standards overview describes three points for this work: in a lab before use, on site at installation, and again during routine maintenance [4]. The same overview says agencies should work to make their data more accurate [4]. Safety Behind the Wheel Foundation encourages every road agency to follow that advice. It dates from 2002, but the lesson holds. A sensor that drifts gives wrong answers with full confidence.
Outdoor equipment lives a hard life. It gets dirty, wears out, and can lose power or its data link. A station that stops updating leaves a gap in the report. Professional systems therefore need a maintenance plan, not just an installation.
Standards also help different systems work together. They let an agency join equipment from several makers into one network [4].
Limits you should keep in mind
- A reading is a point, not a map. Conditions a mile away may differ.
- A reading is the past, not a forecast. Weather can change in minutes.
- Proof of fewer collisions is thin. FHWA’s fact sheet on Arizona’s highway warning system says it may be hard to tell whether such systems cut collisions. It gives the reason: weather events and crashes are both highly random [6].
From sensor to decision: what agencies do with the data
Raw readings become decisions in a few steps. FHWA describes the Maintenance Decision Support System, or MDSS, as a tool that helps winter maintenance managers choose road treatments. Spreading sand, salt or anti icing chemicals adds grip and stops ice from bonding to the road [5].
Traffic managers use road weather data too. They can use it to decide on traffic control and driver warnings. They can lower speed limits with variable speed limit signs, which are signs whose posted limit can change. They can also post messages on dynamic message signs, which are electronic signs beside or above the road [5].
Minnesota feeds road condition data into its message signs. It also places plow camera images in its 511 website, mobile site and app [10]. The case study credits the state’s use of mobile sensor data with safer roads, better mobility, less salt use and lower costs [10].
Some states go a step further. In a strategy called Pathfinder, state transportation departments, National Weather Service offices and weather contractors share forecasts and road conditions. They turn them into one consistent message about likely travel impacts [11]. FHWA says impact based statements help drivers decide whether to travel, and when and where [11].
How drivers get road weather information
Many drivers meet this system through 511. FHWA describes it as a three digit phone number, available nationwide, that gives current travel conditions. The Federal Communications Commission set it aside for travel information on July 21, 2000, for states and local agencies to use [12]. Those agencies run their own services, so details differ. Many also offer websites and apps [1, 10].
FHWA keeps a national page that links to each state’s traffic and road closure information. It was last updated on April 20, 2026 [13]. If you do not know your state’s address, start there.
Comparison compiled by Safety Behind the Wheel Foundation
Where drivers can get road weather information
| Channel | What you can learn | Watch out for |
|---|---|---|
| 511 phone line, website or app | Current travel conditions, sometimes plow cameras [10, 12] | Content differs by state |
| Message signs on the road | Warnings picked from stored messages based on sensor inputs [6] | You see them only once you are on the route |
| Highway advisory radio | Stored radio messages for the area [6] | You must be tuned in and in range |
| Variable speed limit signs | A lower posted limit when conditions worsen [5, 6] | Even the lower limit may be too fast for the weather |
| National Weather Service alerts | Warnings and advisories for your area [14] | Not every alert type reaches phones |
| Map and navigation apps | Routes and closures | May not match the official state report |
Phone alerts need extra care. The National Weather Service page on wireless emergency alerts lists dust storms and snow squalls among the warnings that trigger them. It does not list winter storm warnings or dense fog advisories [14]. So do not wait for your phone to buzz. Check on purpose.
One example: dust, wind and ice on an interstate
Arizona’s Dual Use Safety Technology system, known as DUST, covers stretches of Interstate 10. It detects windblown dust and icy conditions and warns drivers early [6]. Forward scatter visibility sensors read visibility out to more than 10 miles. Wind sensors help predict when high winds may start. Cameras let staff confirm what the sensors report [6].
Alerts reach drivers on message signs and highway advisory radio. On the newer segment, programmable signs can drop the legal speed limit from 75 mph to as low as 35 mph [6]. That is the full chain at work: sensor, decision, message and driver.
What the evidence shows, and where it is thin
The crash figures above show the scale of the problem. They do not prove that a particular sensor system prevents collisions. FHWA’s Arizona summary calls that question open, because both weather and crashes are highly random [6].
Some research looks at driver behavior instead. A 2019 driving simulator study tested a system that sends recommended speeds to drivers as fog thickens. Drivers slowed down in all three zones tested: clear, transition and fog. The effect was stronger in heavy fog than in light fog [15].
A simulator is not a real road, so the study cannot say how many accidents the system would prevent. Still, it supports a simple point. Clear, timely messages can change how fast people drive. Road weather systems are decision aids, and they work only when people act on them.
Tools of your own: who uses what
- Everyday drivers get the most value from free official sources: 511, the state traveler page and weather alerts.
- RV owners and long distance travelers can check each state’s page along the route. Mountain passes and open plains can change from one segment to the next.
- Rural property owners may want a home weather station for their own land. It tells you about your driveway, not the county road in front of it.
- Fleets can build official reports into trip rules. Our guide to journey management shows how.
- Road agencies buy professional systems and must plan for calibration and upkeep.
Auto parts stores and outdoor retailers sell thermometers and home weather stations. Agencies buy road grade stations through roadside equipment suppliers and engineering contractors.
If you buy a personal device, keep these points in mind:
- Check what it measures. Air temperature is not road surface temperature.
- Follow the maker’s advice on where to mount it. Shade, heat and wind change the reading.
- Replace batteries and clean the unit on a schedule.
- Treat the number as a clue. Compare it with the official report.
The most common mistake is trusting one reading. A number taken at your car says little about a shaded bridge deck or a hollow a mile ahead. Another mistake is checking screens while moving.
Public road equipment belongs to the agency that runs the road. Do not install, move or tamper with it. If you see a damaged sign or a dead report, tell the agency instead.
A pre trip routine and questions for your road agency
This routine works for commuters, RV owners and fleet drivers alike. Do it before you start the engine.
- Check the forecast and any weather alerts for the start, the end and the high or low points of your route.
- Open your state’s traveler information page. Read road conditions and closures, and look at cameras if the site offers them.
- Note the spots flagged for ice, fog, dust or wind. Pick safe places where you could pull off.
- Set up your route and alerts while parked. Hand any phone task to a passenger once you are moving.
- Choose your go, delay or detour rule in advance. For example: “If I cannot see the road edge, I leave at the next safe exit.”
- On the road, obey posted variable speed limits and message signs. Drive to what you see, even if the reports look better.
Residents, school board members and small employers can also ask the road agency about its system. Phone or email the traffic operations or maintenance office, and use the checklist below.
Frequently asked questions
Is 511 available everywhere in the United States?
FHWA describes 511 as a three digit number available nationwide, set aside by the Federal Communications Commission in 2000 [12]. What each state or local agency offers differs. Use FHWA’s state directory to find your state’s official page, then see what it provides [13].
Can I put a road weather sensor near my road or at the end of my driveway?
Not on a public road or the land beside it that the agency controls. Road agencies select, install and run that equipment. On your own private land you may use a personal weather station, but treat it as a personal tool. It is not an official report and should not replace one.
Why does my car’s temperature reading differ from the road report?
Your car reads the air near one spot at one moment. A road station may read the pavement itself at a fixed place [2]. Neither reading is wrong. They answer different questions, and the road ahead can differ from both.
Do road weather sensors prevent crashes?
They support decisions that can. Crews use them to time treatments, and traffic managers use them to post warnings [5]. Proof of crash savings is limited, though. FHWA notes that weather and crashes are both highly random, which makes testing hard [6]. A simulator study found slower speeds in fog after drivers got speed advice [15].
How can fleets use this information?
Fleet managers can read the same public reports that drivers use. They can set rules for delaying or rerouting trips when visibility drops or ice is flagged. The state traveler page and weather alerts are good starting points [13, 14].
What should I do if a sign or 511 report looks wrong?
Trust what you see on the road. Slow down and leave more space. Once you are parked safely, tell the agency that runs the road, using its website or phone line. Reports can lag, and a sensor may be down. Your note helps crews fix the problem.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation urges drivers to check official road weather reports before every trip in bad weather and to drive to the conditions they see. Road weather systems turn readings from pavement and air into decisions and warnings. They help crews treat roads and help drivers choose when, where and whether to travel. They work best when the sensors are well placed, calibrated and maintained [3, 4].
No system sees every curve or bridge. Check official sources before you leave, set up your route while parked, and slow down when the road looks worse than the report. If a trip does not feel safe, delay it.
Related articles
- Driving in snow and ice: grip, black ice, skids and when to stay home
- Driving in fog: safe speeds, the right lights and when to pull off
- Journey management for work drivers: weather, breakdowns and check ins
- Rural road safety: why country roads are deadlier and how to drive them
- Using GPS navigation safely: plan the route, skip the sudden moves
Sources and further reading
- 1Federal Highway Administration (FHWA), Road Weather Management Program. Frequently Asked Questions. Page undated. ops.fhwa.dot.gov/weather/faq.htm
- 2FHWA, Road Weather Management Program. Surveillance, Monitoring and Prediction. Page undated. ops.fhwa.dot.gov/…/surveillance.htm
- 3FHWA. Siting Guidelines for Road Weather Information System (RWIS) Environmental Sensor Stations (flyer, FHWA-HOP-05-025). Year not shown in the document. ops.fhwa.dot.gov/…/esssitingguidflyer05.pdf
- 4U.S. Department of Transportation, Road Weather Management Program. An Introduction to Standards for Road Weather Information Systems (RWIS). July 2002. ops.fhwa.dot.gov/…/introstdsrwis.pdf
- 5FHWA, Road Weather Management Program. Decision Support, Control and Treatment. Page undated. ops.fhwa.dot.gov/…/decision.htm
- 6FHWA. Arizona Department of Transportation Dual Use Safety Technology (DUST) Warning System (FHWA-HOP-22-071). 2022. ops.fhwa.dot.gov/…/fhwahop22071.pdf
- 7FHWA, Road Weather Management Program. Snow & Ice. Page undated. Data year: not stated on the page (annual figures). ops.fhwa.dot.gov/…/snow_ice.htm
- 8FHWA, Road Weather Management Program. Low Visibility. Page undated. Data year: not stated on the page (annual figures). ops.fhwa.dot.gov/…/low_visibility.htm
- 9FHWA, Road Weather Management Program. ESS Map & Table. Page undated. Data year: not stated on the page. ops.fhwa.dot.gov/…/essmap.htm
- 10FHWA. Weather Responsive Management Strategies (WRMS): Minnesota DOT Case Study (FHWA-HOP-19-080). Year not shown in the document. ops.fhwa.dot.gov/…/index.htm
- 11FHWA. Weather-Savvy Roads: What is Pathfinder? (FHWA-HOP-18-034). Year not shown in the document. ops.fhwa.dot.gov/…/index.htm
- 12FHWA, Office of Operations. About 511. Page undated. ops.fhwa.dot.gov/travelinfo/about/about511.htm
- 13FHWA. National Traffic and Road Closure Information. Updated April 20, 2026. highways.dot.gov/traffic-info
- 14National Weather Service, NOAA. Weather warnings on the go! Updated January 27, 2022. weather.gov/wrn/wea
- 15Zhao X, Xu W, Ma J, Li H, Chen Y, Rong J. Effects of connected vehicle-based variable speed limit under different foggy conditions based on simulated driving. Accident Analysis and Prevention 128:206-216. Published 2019. PubMed record (DOI 10.1016/j.aap.2019.04.020). Data year: 2019 (driving simulator experiment). pubmed.ncbi.nlm.nih.gov/31055185
This article reflects the safety priorities of road weather monitoring and clear traveler information. It is general education, not legal, medical, insurance, engineering or financial advice. Systems and services change, so verify details with official sources. For your own situation, contact your state or local road agency, your state transportation department's traveler information office, or your fleet's safety manager. For weather alerts, contact your local National Weather Service office. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Road weather sensors and 511: how ice, fog and rain reports reach drivers.” drivewithcare.org, https://drivewithcare.org/road-weather-sensors-and-traveler-information/. Reviewed October 2026.
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