Driving in fog: safe speeds, the right lights and when to pull off
How fast to drive in fog, which lights to use and when to pull off
Picture a two lane road just after sunrise in late fall. As the road dips toward a river, a white wall swallows the taillights you were following. You can suddenly see only a few lane dashes.
From 2019 to 2023, low visibility crashes averaged about 32,900 a year, injuring about 11,300 people and killing 488, the Federal Highway Administration (FHWA) reports [1]. Its low visibility category includes fog, smog, smoke, blowing snow and blowing sand or dirt [1].
These crashes were only 4% of weather related crashes but 13% of weather related deaths [1]. Per crash, that makes them nearly 3 times as deadly as weather related crashes overall (our calculation from FHWA figures).
This road weather guide covers where fog forms, how it fools you, and how to set your speed, lights and stopping point.
Where and when fog forms
Fog is a cloud resting on the ground. It forms when air near the surface cools to its dew point, the temperature at which water vapor condenses into droplets [3]. The National Weather Service (NWS) names 3 main types: radiation, advection and evaporation fog [2].
- Radiation fog forms on clear, calm nights as the ground loses heat and chills the air above it. Cool air drains downhill, so valleys fog up first. The fog usually lifts as the sun warms the air or wind picks up [2, 3].
- Advection fog forms when wind pushes warm, moist air over a colder surface, often along coasts. It may not burn off in the morning and can linger for days [3].
- Evaporation fog, such as steam fog, forms when cold, dry air moves over warmer water, typically in the coldest months [3].
When surface temperatures are at or below freezing, fog droplets can freeze on roads. NWS may issue a Freezing Fog Advisory [4]. Super fog forms when smoke from damp, smoldering brush mixes with moist air, cutting visibility to a few feet [2].
NWS issues a Dense Fog Advisory when widespread fog cuts visibility to a quarter mile or less [4]. It warns that patchy fog can change visibility suddenly and urges extra care on bridges and in valleys [2]. Road weather specialists watch for clear, calm evenings when the temperature is sliding toward the dew point, a setup that often means fog in low spots by sunrise [3].
Why fog fools your eyes
Distance: In fog chamber experiments simulating night fog, people judged vehicles shown by their rear lights to be about 60% farther away than in clear air [5]. Errors grew when a vehicle showed only 1 rear fog light or lights set close together [5]. The lab fog was extremely dense, but the lesson holds: taillights that seem far ahead may be much closer.
Speed: A 2012 driving simulator study found that people underestimated their speed when contrast faded evenly across the whole scene [6]. In realistic fog, where distant objects fade more than nearby ones, they overestimated their speed and slowed down [6]. Because findings differ, treat your sense of speed in fog as unreliable and check the speedometer.
Traffic: FHWA lists fog among the conditions that change speeds, widen the spread between fast and slow drivers and raise crash risk [1]. Yet average freeway speeds drop only about 10% to 12% in low visibility [1]. When visibility falls much further than speeds do, many drivers are outrunning what they can see.
Speed: stop within the distance you can see
NWS and California’s driver handbook share one core rule, which our table turns into numbers: drive slowly enough to stop within the distance you can see [2, 10].
Comparison compiled by Safety Behind the Wheel Foundation
Highest speed that lets you stop within your sight distance
| You can see about | White lane dashes visible | Alert driver (1.5 second reaction) | Cautious plan (2.5 second reaction) |
|---|---|---|---|
| 50 feet | about 1 | 14 mph | 10 mph |
| 100 feet | 2 to 3 | 22 mph | 18 mph |
| 150 feet | about 4 | 29 mph | 24 mph |
| 200 feet | 5 | 35 mph | 30 mph |
| 300 feet | 7 to 8 | 45 mph | 39 mph |
| 400 feet | 10 | 54 mph | 48 mph |
| 600 feet | 15 | 68 mph | 62 mph |
| 1,320 feet (a quarter mile) | about 33 | Above most speed limits | Above most speed limits |
Calculated by Drive With Care and rounded down: reaction distance plus braking distance on a level road. Braking uses the rate road designers assume, 11.2 feet per second per second [7]. FHWA puts normal reaction times at about 0.75 to 1.5 seconds. Designers allow 2.5 seconds [7]. Ice, snow, downhill grades, worn tires or an unlit object call for slower speeds.
How to estimate what you can see: On most highways, broken lane lines are 10 foot dashes separated by 30 foot gaps [8]. Each dash plus its gap covers about 40 feet, so seeing 5 dashes ahead means roughly 200 feet (our calculation). Spacing varies on some roads, so treat the count as a rough guide [8].
Plan on the cautious column: at 60 mph, a stopped car 200 feet ahead gives you about 2.3 seconds (our calculation). That is less than the 2.5 seconds designers allow for reacting alone [7].
Lights: low beams on, high beams off
- Use low beams, day or night: NWS recommends them because switching them on also lights your taillights, so you are visible from both directions [9]. Never use high beams in fog, since their light reflects off the droplets and creates glare [9, 10].
- Don’t count on daytime running lights or “auto” mode: New York’s driver manual states that daytime running lights do not count as headlights [11]. Confirm your dashboard’s headlight symbol is lit, or switch to low beams yourself.
- Add fog lights to low beams, never in place of them: California warns never to drive using only parking or fog lights [10]. New York’s manual notes that switching fog lights off once visibility improves cuts glare for other drivers [11].
Following distance: let the taillights go
In fog, drivers often close the gap to keep the taillights ahead in view. Call it the taillight trap: you trade away your stopping room to follow someone who may brake, turn or run off the road.
- Add room: NWS and California both tell drivers to increase following distance in fog [9, 10]. Our following distance guide adds 1 to 2 seconds for fog. So plan on at least 5 seconds at highway speeds.
- Let the car ahead disappear: At a speed that lets you stop within your sight distance, you don’t need its taillights.
- Stay in your lane: Avoid passing or changing lanes unless it is truly necessary [2, 10].
- Protect your rear: Tap your brakes lightly before slowing so your brake lights warn the driver behind you.
Lane markings, rumble strips and other in fog habits
- Track the right edge line: NWS advises following the lines on the road with your eyes [9]. On two lane roads, the right edge line keeps you farthest from oncoming traffic. Still, stay centered, since cars may be stopped on the shoulder.
- Let rumble strips guide you: If you hear a roar or feel a buzz from grooves along the edge, ease gently back into your lane.
- Keep the glass clear by running your wipers and defroster as needed [10].
- Turn off cruise control, as NWS advises in dense fog [2], so your foot stays ready to brake.
- Use your ears: Lower the radio and listen for traffic you can’t see, as California advises [10].
The Drive With Care visibility decision ladder
Comparison compiled by Safety Behind the Wheel Foundation
| If you can see about | Lane dashes | What to do |
|---|---|---|
| More than a quarter mile | 33 or more | Proceed with low beams on. Expect sudden patches, especially in valleys and on bridges. |
| 600 feet to a quarter mile | 15 to 33 | Keep to about 60 mph or less. Turn cruise control off, skip passing and keep at least 5 seconds of following time. |
| 200 to 600 feet | 5 to 15 | Slow to the cautious speeds in our table, about 30 to 60 mph. Pick your next exit or parking lot now. |
| 100 to 200 feet | 2 to 5 | Leave the road at the next safe exit, rest area or parking lot, and wait. |
| Under 100 feet, or you lose the edge line | Under 2 | Get completely off the road as soon as you safely can, using the steps below. |
Drive With Care guidance based on our stopping distance table, not an official or legal standard.
Why exit at 200 feet? Below that point, a safe speed drops to about 30 mph or less (our calculation). But some drivers around you may keep going much faster. That spread of speeds is part of what makes fog dangerous [1]. Exit while you can still see the ramp. Leave sooner if you catch yourself steering by taillights, gripping the wheel or tiring.
How to pull off safely when fog gets too thick
NWS and California both advise pulling off and waiting when fog becomes too thick to drive safely [9, 10].
- Head for a refuge: The best place to stop is off the highway entirely, such as a business parking lot [9].
- Slow gradually: Check your mirror, signal and turn on your hazard flashers as you slow down [9].
- Get fully off the road: If no lot is available, pull as far off the road as you can, never stopping in a travel lane [9].
- Set up the car: Leave only your hazard flashers on, set the parking brake and take your foot off the brake pedal [9]. NWS warns that steadily glowing rear lights can lead other drivers to run into you [9].
- Wait it out: Radiation fog often lifts as the day warms [3], so recheck conditions before pulling back out.
If you can’t see at all: Don’t stop in your lane. Creep forward with low beams and flashers on, guided by the edge line or rumble strip, until you can get completely off the road.
If your car breaks down in dense fog: Move it to the shoulder and leave only the hazard lights on, NWS suggests [2]. Then walk away from the road if you can [2], and see our roadside breakdown guide.
Smoke, dust, heavy rain and snow squalls
Wildfire smoke: California’s handbook treats heavy smoke like fog: postpone the trip if you can, or slow down, use low beams and add following distance [10].
Dust storms: The Arizona Department of Transportation (ADOT) campaign Pull Aside, Stay Alive gives steps that differ from fog advice [12]:
- Never drive into a dust storm. If one reaches you, check traffic and start slowing.
- Pull completely off the paved roadway. Don’t stop in a travel lane or on the shoulder.
- Set the parking brake, turn off all lights including your flashers and take your foot off the brake.
- Keep your seat belt fastened and wait for the storm to pass.
ADOT cites a 2013 dust storm crash of 19 vehicles on Interstate 10 near Picacho Peak, Arizona, that killed 3 people and injured about a dozen [13].
Comparison compiled by Safety Behind the Wheel Foundation
Stopping in fog vs. a dust storm
| Step | Fog (NWS) | Dust storm (ADOT) |
|---|---|---|
| Where to stop | A parking lot if possible; otherwise as far off the road as you can | Completely off the pavement, not on the shoulder |
| Lights once stopped | Hazard flashers only | Every light off, including flashers |
| Brakes | Parking brake on, foot off the pedal | Parking brake on, foot off the pedal |
| Then | Wait for visibility to improve | Stay buckled in your car until the storm passes |
Why the difference? ADOT’s steps assume your car sits entirely off the pavement, where the bigger danger is a driver steering toward your lights and hitting you [13]. NWS fog advice allows for a car stopped at the roadside that still needs to be seen [9]. So follow the guidance for the hazard you face.
Heavy rain: A downpour or road spray can blind you as quickly as fog. So use the same habits and see our rain and hydroplaning guide.
Snow squalls: NWS describes squalls as short bursts, typically under an hour, that bring sudden whiteouts and can ice roads within minutes [14]. If a Snow Squall Warning covers your area, NWS advises delaying travel until it passes [14]. If you’re already driving, slow down, turn on your headlights and hazard lights and leave plenty of room [14]. Avoid slamming the brakes, which can trigger a chain reaction crash [14].
Before you leave: a fog checklist
Advice for new drivers and older drivers
Frequently asked questions
Should I use high beams or low beams in fog?
Use low beams, because high beams reflect off fog droplets and create glare [9].
How fast should I drive in fog?
Drive slowly enough to stop within the distance you can see [2]. With 200 feet of visibility, plan on about 30 mph. With 100 feet, stay under 20 mph (our calculation).
Should I turn on my hazard lights in fog?
NWS fog guidance calls for flashers as you pull off and once you have stopped [9]. For snow squalls, NWS also suggests them while moving [14]. Check your state’s manual for its rules on flashers while driving.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
In fog, drive at a speed that lets you stop within what you can see. Steer by the road’s markings with low beams on. When visibility drops below about 200 feet, get off the road and wait. Often the safest fog trip is the one you start an hour later.
Related articles
- Driving in rain: hydroplaning, standing water and flood safety
- Driving in snow and ice: grip, black ice, skids and when to stay home
- Safe following distance: the 3 second rule and when to add more
- Night driving safety: how to see hazards and stop in time after dark
- Roadside breakdown safety: what to do when your car breaks down
Sources and further reading
- 1Federal Highway Administration (FHWA), Road Weather Management Program. How Do Weather Events Affect Roads? Five-year averages, 2019 to 2023, based on NHTSA data as reported in May 2024. Accessed October 2026. ops.fhwa.dot.gov/weather/q1_roadimpact.htm
- 2National Weather Service (NWS), Weather-Ready Nation. Social Media: Fog Safety. March 3, 2026. weather.gov/wrn/fog-sm
- 3Federal Aviation Administration (FAA). Pilot’s Handbook of Aeronautical Knowledge (FAA-H-8083-25C), Chapter 12: Weather Theory. Accessed October 2026. faa.gov/…/14_phak_ch12.pdf
- 4NWS. Fog Watches, Warnings and Advisories. Page dated March 28, 2018. weather.gov/safety/fog-ww
- 5Cavallo V, Colomb M, Doré J. Distance Perception of Vehicle Rear Lights in Fog. Human Factors 43(3):442 to 451. 2001. PMID 11866199. pubmed.ncbi.nlm.nih.gov/11866199
- 6Pretto P, Bresciani JP, Rainer G, Bülthoff HH. Foggy Perception Slows Us Down. eLife 1:e00031. 2012. PMID 23110253. elifesciences.org/articles/00031
- 7FHWA. Speed Concepts: Informational Guide, Chapter 4: Engineering and Technical Concepts. FHWA-SA-10-001. Accessed October 2026. highways.dot.gov/…/chapter-4-engineering-and-technical
- 8FHWA. Manual on Uniform Traffic Control Devices, 11th Edition, Part 3: Markings, Section 3A.04. December 2023. mutcd.fhwa.dot.gov/…/part3.pdf
- 9NWS. Fog Safety: Driving in Fog. Page dated March 28, 2018. weather.gov/safety/fog-driving
- 10California Department of Motor Vehicles. California Driver’s Handbook, Section 8: Safe Driving. Updated June 11, 2025. dmv.ca.gov/…/safe-driving
- 11New York State Department of Motor Vehicles. Driver’s Manual, Chapter 10: Special Driving Conditions. Accessed October 2026. dmv.ny.gov/…/chapter-10-special-driving-conditions
- 12Arizona Department of Transportation (ADOT). Never Drive Into Dust Storms: Pull Aside, Stay Alive (news release). June 8, 2026. azdot.gov/news/never-drive-dust-storms-pull-aside-stay-alive
- 13ADOT. Drivers: Do You Know What to Do When Confronted With a Dust Storm? (news release). June 10, 2019. azdot.gov/news/drivers-do-you-know…
- 14NWS. Snow Squalls. Page dated April 15, 2022. weather.gov/safety/winter-snow-squall
- 15Centers for Disease Control and Prevention (CDC). Older Adult Drivers. Last reviewed January 28, 2026. cdc.gov/older-adult-drivers/about
This article reflects the safety priorities of road weather science and driving in low visibility. It is general education, not legal, medical or professional advice for any individual. Figures marked as our calculations are simple estimates that do not predict any specific vehicle’s performance. Laws, forecasts, data and vehicle features change, so verify details with official sources. Reviewed October 2026.
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